Swayambhu

Swayambhu () is a Sanskrit word that means "self-manifested", "self-existing", or "that is created by its own accord". 
Often, the word swayambhu is used to describe a self manifested image of a deity, which was not made by human hands, but instead is naturally arisen, or generated by nature.
According to the Ramayana and mahabharata Vishnu called swayambhu.
The word etymology of swayambhu is 'Svayam' () which means 'self' or 'on its own' and 'bhu' () which means 'to take birth' or 'arising'. This is applicable to the physical and tangible idols of Gods that we see or to the intangible yet existing God whom we cannot see. For example, the jyotirlingas are considered swayambhu- the idol of Venkateshwara at Tirumala and some other ones are considered Swayambu- which means they are not installed by any person but they exist on their own. The word Swayambhu is also applicable to Gods- for example- Vishnuis described as swayambhu in Ramayana, Vishnu is called swayambhu in Matsya purana.
Vaishnavism.
Based on details in Bhagavata Purana and Matsya Purana, Narayana or Krishna is said to be the self-manifested svayambhu form of Brahman as the first cause of creation. (See Hiranyagarbha)
The Ice Lingam at Amarnath Cave.
Inside the high Amarnath cave, the stalagmite is formed due to freezing of water drops that fall from the roof of the cave on to the floor and grows up vertically from the cave floor. It is considered to be a Shiva Linga by Hindus. The Cave waxes during May to August, as snow melts in the Himalayas above the cave and the resultant water seeps into the rocks that form the cave and gradually wanes thereafter. As per the religious beliefs, it has been claimed that the lingam grows and shrinks with the phases of the moon reaching its height during the summer festival.
References.
1 "Amarnathji Yatra - a journey into faith". Official Web Site of Jammu and Kashmir Tourism.
4 "Stalactites and Stalagmites - Cave, Water, Caves, and Growth - JRank Articles". Science.jrank.org. Retrieved 2013-04-15.
Jump up ^ Ortner, Jon. 5 On the road again. PDN Gallery.