Shaligram

Shalagrama(शालग्राम) or Shaligram is a fossilized shell used in South Asia as an iconic symbol of the god Vishnu as the Universal Principle by Hindus of Vaishnavite and Smarthist sects.Shaligrams are usually collected from river-beds or banks of the Gandaki River in Nepal at Muktinath. They are considered easy to carry and popular in certain traditions of Vaishnavism, as an aniconic representation of the divine. They are typically in the form of spherical, black-coloured Ammonoid fossils of the Devonian-Cretaceous period which existed from 400 to 66 million years ago.
Legend.
According to the Devi Bhagavata Purana, Brahmavaivarta Purana & Shiva Purana, Shalagrama Shilas originated due to the following chain of events. 
A king named Vrishadhvaja had been cursed by Surya to endure poverty due to his reluctance to worship any deity other than Lord Shiva. To regain back the lost prosperity, his grandsons Dharmadhvaja & Kusadhvaja performed austerities to propitiate Goddess Lakshmi, the presiding deity of prosperity. Pleased with the austerities, She granted them prosperity & boon of being born as their daughters. Accordingly Lakshmi incarnated as Vedavati, the daughter of Kusadhvaja & Tulasi, the daughter of Dharmadhvaja. Tulasi went to Badarikashrama to perform austerities in order to gain Lord Vishnu as her husband, but was informed by Lord Brahma that she wouldn't get Vishnu as her husband in that life, rather she would have to marry the Danava named Shankhachuda. 
In his previous birth, Shankhachuda was Sudama, an attendant of Krishna, a manifestation of Vishnu residing in Goloka, created out of his body who was cursed by Radha to be born as a Danava. As a result, Shankhachuda was virtuous & pious by nature and he was devoted to Vishnu. He married Tulasi on the command of Lord Brahma as per the rules of the Gandharva marriage. After Shankhachuda's marriage, the Danavas under his leadership waged a battle against their natural enemies, the Devas, in which they won due to merit of Shankhachuda's virtue. The Devas were subsequently driven out of Svarga by the victorious Danavas.
Demoralised & defeated, the Devas approached Lord Vishnu, who told them that Shankhachuda was destined to be killed by Lord Shiva. On being requested by the Devas, Lord Shiva along with His attendants & the Devas, waged a battle against the Danavas led by Shankhachuda. However, neither side was able to outpower the other. An unembodied voice told Lord Shiva that Shankhachuda was invincible in combat as long as he wore his armour & his wife's chastity wasn't violated by the boon of Lord Brahma. 
So Lord Vishnu, assuming the form of an old brahmin, asked from Shankhachuda his armour in alms. Shankhachuda donated his armour to him. When he was busy fighting, Lord Vishnu,wearing Shankhachuda's armour, assumed the form of Shankhachuda & cohabited with Tulasi. Thus Tulasi's chastity had been broken & Shankhachuda was killed by Lord Shiva's trident, thereby reliving Sudama from the curse. 
At the moment of Shankhachuda's death, Tulasi became suspicious that the man who was with her at that time wasn't Shankhachuda. When she came to know that it was Lord Vishnu who had violated her, she cursed Him to become a stone as He had been stony-hearted in accomplishing the death of his devotee Shankhachuda & violating her, who was also devoted to Him. Lord Vishnu consoled Tulasi by stating that it was the result of her austerities performed in the past in order to gain him as her husband, & that she would again become his wife on casting off her body. Thus Lakshmi cast off the body of Tulasi & assumed a new form (which became known by the name of Tulasi). The discarded body of Tulasi was transformed into the Gandaki river, & from her hairs emerged the tulsi shrub. Lord Vishnu, on being cursed by Tulasi, assumed the form of a large rocky mountain known as Shalagrama, on the banks of the Gandaki river where "vajrakita", a type of worms having teeth as strong as vajra carve out various markings on his body. The stones carved by "vajrakita" which fall down from the surface of that mountain, become known as Shalagrama shilas.
History.
Shaligramas are mostly black coloured stones with marks, and are the fossilized remains of now extinct sea dwelling ammonites. Hence they are found in river beds and other regions that were once underwater, the most popular being the Himalayas and Nepal. Historically, the use of Shaligrama (or Salagrama) Shilas in worship can be traced to the time of Adi Shankara through the latter's works. Specifically, his commentary to the verse 1.6.1 in Taittiriya Upanishad and his commentary to the verse 1.3.14 of the Brahma Sutras suggest that the use of Saligrama in the worship of Vishnu has been a well-known Hindu practice. A good number of false shaligrams too remain in circulation. 
The statue of Lord Vishnu in the Padmanabhaswamy Temple of Thiruvananthapuram & Badrinath Temple of Garhwal region, & that of Lord Krishna in Krishna Matha of Udupi & Radha Raman Temple of Vrindavana are also believed to be made from the Shaligrams.
Use.
Although Hinduism has many anthropomorphic murtis (images) of gods, aniconism is also represented with such abstract symbols of God as the Salagrama
Configurations.
Quoting from sources like "Viramitrodaya", "Chaturavarga-chintamani", "Puranasamgraha" etc the "Pranatoshani Tantra" cites the following varieties of Shalagrama Shilas on the basis of shape, colour, features of the "chakra"(imprint of the ammonite shell present on the outer surface or inside the stone, resembling Sudarshana Chakra, the main weapon of Vishnu) & "mukha/vadana/dvara"(a large aperture resembling an open mouth through which the "chakra" present within the stone can be seen from outside) & distinct markings like the "vanamala"(a linear white line running throughout the body of the stone, resembling a garland) 
● Keshava: i) Marked with a single "chakra", a "vanamala" and 4 "bindu"s(circular markings present on the outer surface) of golden hue arranged like a square. ii) Marked with a shankha and "chakra" on the lower middle portion. Grants prosperity & fulfilment of all desires on being worshipped.
● Hayagriva: i) Blue in colour at the top, shaped like an elephant-goad or the head of a horse, marked with a linear mark present near the "chakra" and several "bindu"s. ii) Marked with 5 linear marks, other characteristics being the same before. iii) Marked with a single "chakra" and a marking resembling a banner. iv) Has both yellow & red colours on it's surface, endowed with 2 "chakra"s, marked with markings resembling ear-rings & gems on the sides, shaped like an elephant-goad or lotus-bud. v) Shaped like a lotus-bud while the "mukha"shaped like the head of a horse, and marked with a marking resembling a rosary. vi) Green in colour, marked with a single "chakra", the left side is elevated like that of a kapinjala bird. Grants knowledge, prosperity & wordly enjoyment on being worshipped. 
● Parameshthin: i) Presence of a hole(which doesn't reaches the other side of the stone, otherwise the stone would be unfit for worship) at the top, has a single "chakra", marked with the markings resembling a lotus and several "bindu"s. ii) White in colour, has a single "chakra" & markings resembling a lotus, shaped like a snake's hood/pearl necklace/sphere, the top is perforated & hard. iii) Reddish in colour, circular in shape with a single "chakra" and linear mark, perforated & hard. iv) Round in shape, yellow in colour with a hole at the top. v) Red/white in colour while the top surface is yellowish & perforated, marked with a single "chakra" & markings resembling a lotus. Grants enjoyment & liberation on being worshipped. 
● Hiranyagarbha : i) The colour is yellow like that of honey and slightly elongated in shape, has several golden linear marks on the body with a crystal-like glow & shaped like the round moon. ii) Black in colour, round in shape, the "vadana" is glazed, marked with a single "chakra" & marking resembling "srivatsa"(a triangular mark on Vishnu's chest) on a of the top surface. Grants prosperity & increase in progeny on being worshipped.
● Chaturbhuja : The colour is blue like that of a rain cloud. It is round in shape & marked with 4 "chakra"s. Grants fearlessness on being worshipped.
● Gadadhara : Circular in shape, yellow in colour, the "chakra" is situated towards the left, the middle part is marked with 3 linear marks, markings resembling a banner, a "vajra" & elephant-goad are present on it. 
● Narayana :i) "Shyama"(dark colour of lighter tone) in complexion, the "chakra" is situated at middle portion, marked with a long linear mark, the right side is perforated. ii) The "vadana" is situated at the middle of the body, within which lies the "chakra" at the front, endowed with markings resembling ornaments like ear-rings, bracelets & nekclaces. Grants all sorts of success on being worshipped.
● Madhava : With a colour like that of honey, marked with a gada and shankha, the "chakra" is situated at the middle & is glazed. Grants liberation on being worshipped.
● Govinda : i) Black in colour, shining in appearance, endowed with a single "chakra" alongside markings resembling a gada on that right side and that resembling a mountain on the left. ii) Black in colour, not much large in size, the central portion is raised upwards, endowed with a large "vadana" marked with 5 "chakra"s situated towards the left
● Vishnu : Black in colour, possesses a large "chakra", shaped like a gada, a linear mark is present on the top surface near the "vadana". Grants liberation on being worshipped.
● Madhusudhana : Golden in colour with shining appearance, has a single "chakra" & endowed with markings resembling a lotus & a shankha. Destroys enemies on being worshipped.
● Trivikrama : "Shyama" in colour, triangular in shape woth shining appearance, has 1 or 2 "chakra"s alongwith some "bindu"s on the left-side & a linear mark on the right-side . Grants wealth on being worshipped.
● Shridhara : i) The shape is as round like the kadamba flower, endowed with "vanamala" & 5 linear marks at the top surface & sides. ii) Green in colour, round in shape with a flat upper surface, endowed with a single glazed "chakra" & markings resembling a lotus in the middle. iii) Small in size, has 2 "chakra"s & a "vanamala". iv) Glittering like a gem, endowed with a single "chakra", "vanamala", markings resembling a lotus/banner & another resembling an elephant-goad near the "vadana". Grants prosperity to householders on being worshipped.
● Hrishikesha : Shaped like a half moon with a single "chakra" and also with markings resembling the hair of a boar. 
● Padmanabha : Reddish in colour, endowed with a semicircular "chakra" & markings resembling a lotus & hairs of a mane. Causes poverty & sorrow, hence shouldn't be worshipped.
● Damodara : i) Big in size, the colour is as green as durva grass with a small "chakra" situated in the middle, a small "vadana" & a single yellowish linear mark in the middle ii) Endowed with 2 "chakra"s & a cracked outer surface. Grants auspiciousness on being worshipped.
● Sudarshana : i) Green in colour with gliterring appearance, the "chakra" & markings resembling a gada are situated on the left side while linear markings arranged like a lotus are found on the right side. ii) Black in colour, shaped like a lotus, endowed with a single "chakra", a large "vadana" & lowered middle portion. Destroys all sins & grants results of all types of worship the on being worshipped.
● Vasudeva : White in colour, endowed with a single or 2 "chakra"s at the "dwara". Fulfills all desires on being worshipped.
● Pradyumna : i) Yellow in colour, has a small "chakra" & markings resembling a makara on the sides & top surface, endowed with many holes. ii) The colour is as blue as rain-clouds, has a small "chakra" & small "mukha" & many holes. Grants prosperity & happiness to householders on being worshipped.
● Aniruddha : i) Blue in colour, round in shape and glazed, has markings resembling a lotus at the top surface and 3 linear marks near the "dvara". ii) Black in colour with a beautifully shaped "dwara" and having a "chakra" near the centre, another on the sides and a small "chakra" at the top surface. iii) Yellow in colour, round in shape. Grants happiness to householders on being worshipped.
● Purushottama : i) Golden in colour with a "chakra" at the middle portion and a bigger "chakra" at the front. ii) The colour is as yellow as the atasi flower and endowed with many "bindu"s. iii) With "mukha"s on all directions (traditionally numbered to ten). Increases prosperity & grants enjoyment and liberation on being worshipped.
● Adhokshaja : Dark black in colour with patches of brown, circular in shape, endowed with a single "chakra" & red linear marks, maybe large or small in size. Grants auspiciousness to worshippers on being worshipped.
● Achyuta : Has four "chakra"s on right and left sides and 2 red markings resembling ear-rings at the "mukha", also marked with markings resembling shankha, gada, bow, arrow, pestle, banner, white umbrella and a red elephant-goad.
● Upendra : Green in colour with glittering appearance, has a glazed body with one or more "chakra"s on the sides.
● Janardana : Has 2 "dwara"s; one on the eastern side & another on the western side & 4 "chakra"s; 2 on the top surface & other 2 in the middle. Grants prosperity & destroys enemies on being worshipped.
● Lakshmijanardana : The colour is as blue as rain-clouds with one "dwara" & 4 "chakra"s.
● Hari : Green in colour, round in shape with one "mukha" at the top, the lower portion is marked with "bindu"s. Fulfills all desires on being worshipped.
● Ananta: i) Marked with markings resembling the hood of a snake, has 20 red "chakra"s, 14 "dwara"s & many "bindu"s, has different colours on it's bod. ii) The colour is as blue as a rain-cloud, circular in shape, has 27 "chakra"s. Grants 4 aims of human life & results of all types of worship on being worshipped.
● Yogeshwara : Has a shivalinga like structure on it's top surface. Grants redemption from the great sin of "brahmahatya" on being worshipped.
● Pundarikaksha : Has markings resembling a pair of eyes/lotuses on the left or top surface or sides. Can bring the entire world under control of the worshipper on being worshipped.
● Chaturmukha : Has 4 linear marks on the sides, and 2 "chakra"s on the middle portion of the body.
● Yajnamurthi : Has both yellow & red colours on it's surface, with a small "dwara" and 2 "chakra"s; one at the middle and other on the right side.
● Dattatreya : i) Has white, red and black patches and markings resembling a rosary on the top surface. (Some versions state red & yellow colours)
● Shishumara : Long in shape, with a deep triangular opening and having one or two "chakra"s on the front side and another on the back side. Grants all sorts of success on being worshipped.
● Hamsa : Shaped like a bow having both blue and white colours on it's surface and has a "chakra" and markings resembling a lotus. Grants only salvation on being worshipped.
● Parahamsa : The colour is blue like the throat of a peacock, with a glazed body and round "dwara" within which lies a single "chakra" & a glittering patch resembling the sun on the right side of the "chakra". There are also 2 linear marks forming the shape of a boar on the body. Grants 4 aims of human life on being worshipped.
● Lakshmipati : Black in colour while the sides or the front portion is blue like a peacock's throat, has a small "chakra" & large "vadana". Grants prosperity & wealth on being worshipped.
● Garudadhvajalakshmipati : Round in shape, has golden marks resembling horns and hoofs on the body & a smooth "chakra" with black linear marks on it.
● Batapatrashayin : Round in shape with white, copper-red and blue colours on it's surface, has a single "vadana" in the middle, 4 "chakra"s & 3 "bindu"s, markings resembling a shankha & a lotus lie to the left & right side of the "chakra" respectively. 
● Vishvambhara : Has 20 "chakra"s on the body.
● Vishvarupa : Circular in shape, has 1 or 5 "dwara"s and many "chakra"s. Bestows children & grandchildren on being worshipped.
● Pitambara : Round like the udder of a cow or a human breast, has a single "chakra". Grants happiness on being worshipped.
● Chakrapani : Round in shape and glazed, with a small "chakra" and many other prints.
● Saptavirashrava : Round in shape with a small "chakra" and several golden "bindu"s all over the body. Increases all sorts of prosperity on being worshipped.
● Jagadyoni: A single red "chakra" is present within the "dwara". Grants auspiciousness on being worshipped.
● Bahurupin : With multiple "mukha"s & single "chakra" alongside markings resembling a shankha present inside. Grants salvation on being worshipped.
● Harihara : Has 4 "dwara"s & 2 "chakra"s with a "shivanabhi"(a special form of shivalinga) like structure on it's top surface. Grants prosperity & happiness on being worshipped.
● Swayambhu : Blue in colour with a long and big "mukha", and having the body encircled by linear marks. Grants only salvation on being worshipped.
● Shivanarayana : Has 2 "mukha"s & 4 "chakra"s. Destroys wealth, property & progeny, hence shouldn't be worshipped.
● Shankaranarayana : Has a "shivanabhi" like structure either sideways or left side or right side. 
● Pitamaha : Has 4 different "dwara"s with a "chakra" in each of them.
● Naramurti : The colour is yellow like the atasi flower with markings resembling a sacred thread on the sides.
● Shesha : Printed with linear marks forming the coiled body of a snake. Although red in colour, it isn't considered inauspicious. 
● Pralambaghna : Red in colour with the markings resembling the coiled body and hood of a snake. Causes death, hence shouldn't be worshipped.
● Suryamurti : Has 12 different "chakra"s either on the body surface or inside the "dwara". Destroys illnesses on being worshipped.
● Haiheya : Has one "mukha" and mutiple markings resembling hoods, out of which 2 are found on the right side of the "dwara", shaped like a lotus petal with a golden mark resembling an arc. Grants all sorts of success on being worshipped.
● Vishnupanjara : Printed with several linear marks.
● Garuda : i) Shaped like a lotus with three marks one above the other the central line being longer, has 4 "chakra"s. ii) Printed with markings resembling a pair of wings and having two, three or four golden linear marks on his body, green, blue or white in colour. Destroys all sins on being worshipped.
● Matsya : i) Elongated in shape, the colour is like gold/bell-metal, and marked with three "bindu"s. ii) Elonhated in shape resembling a fish, green in colour with crystal-like glow, endowed with 2 "chakra"s at the middle portion & 3 "bindu"s. iii) Has 3 elongated "dwara"s each possessing a "chakra" while another "chakra" is situated at the posterior end shaped like the tail of a fish, the right side is shaped like a cart, the left side has a linear mark. iv) Has a long "dwara" at the right side, endowed with 3 "bindu"s, a single "chakra" & markings resembling a shankha & lotus
v) Shaped like a fish & a has markings resembling an elongated fish. Grants enjoyment, liberation, fulfilment of all desires & auspiciousness on being worshipped.
● Kurma : i) Shaped like a tortoise with the eastern side elevated. ii) Green in colour, round in shape resembling a tortoise, the top surface being elevated and endowed with red markings resembling Kaustubha, 5 circular markings resembling the sun & a "chakra". iii) Endowed with structures resembling the feet of a bed on the sides. iv) Endowed with a single "chakra", 3 golden "bindu"s & markings resembling a shankha & a lotus. v) Elongated in shape with the "mukha" extending from left to right side, and endowed with 5 circular markings resembling the sun. vi) Triangular in shape like the inflorescence of a snuhi plant with "chakra"s on both sides. vii) Round in shape resembling a tortoise, has both blue & red colours on it's surface, has a long "dwara" & 2 "chakra"s imprinted sideways at the middle portion of the body. Fulfills all sorts of desires & increases progeny on being worshipped.
● Varaha : i) Blue in colour, big in size, and printed with odd number of "chakra"s & three linear marks. ii) Endowed with even number of "chakra"s, of which at least one is situated on the right side, and "vanamala". This last variety is called Lakshmi-Varaha. Grants enjoyment and liberation on being worshipped.
● Narasimha : Has a very large "mukha", 2 "chakra"s & linear marks resembling the mane of a lion. Creates detachment in the mind of the worshipper on being worshipped.
● Vamana :
i) Round in shape like the kadamba flower, small in size and marked with 5 linear marks. ii) Small in size with glittering appearance, the shape is circular or triangular like the inflorescence of the snuhi flower, has a "chakra" on both top and bottom surface with markings resembling the Brahminy kite beside the "chakra". iii) Not very small in size, has a single prominent glazed"chakra" at the centre. iv) The colour is yellow like the atasi flower with an elevated top surface, white "bindu"s at the "mukha" & an indistinct "chakra". v) The colour is blue like the rain-cloud, round like the bilva fruit or seeds of the jujube fruit in shape, has 2 "chakra"s, a "vanamala" & a small "mukha". This last variety is called Dadhivamana. Grants fulfilment of all desires, happiness, prosperity; increase in property, progeny & wealth of householders on being worshipped.
● Parashurama : Yellow or black in colour and marked with markings resembling an axe, has a "chakra" on either left or right side & markings resembling teeth, either at the top surface or on the sides. Prevents untimely death on being worshipped.
● Ramachandra : The colour is green like the durva grass, endowed with a single "chakra", markings resembling a staff on the top surface & 2 linear marks on the sides. 
● Krishna : i) Black in colour, endowed with a single "chakra" at the "dwara" & a "vanamala". ii) Has yellow patches, the "dwara" is situated at the middle, the top surface resembles a tortoise shell, endowed with yellow "bindu"s at the sides. Grants wealth, crops, happiness & redemption from sins on being worshipped.
● Shankarshana : Red in colour, has 2 "chakra"s joined with each other on the top side, the eastern side is perforated. Grants happiness to householders on being worshipped.
● Buddha : Has a very small "mukha" but without any "chakra". It is also called "Nivita"(hidden) "Buddha". Grants the ultimate position("parama pada") on being worshipped.
● Kalki : i) The colour is black like that of a bee, has 6 "chakra"s & markings resembling a sword/dagger over the "mukha" on the top surface. ii) Shaped like the face of a horse and marked with 3 "chakra"s. Destroys all evil caused by "kali yuga" on being worshipped.
More detailed classification is provided in the book "Shalagrama Kosha".