a main material of the fountain pen nib of the present invention is corundum including sapphire glass and ruby , topaz , emerald , or garnet , preferably corundum including sapphire glass and ruby , and more preferably sapphire glass . the main material may be a natural product or a synthetic product , and may be a single crystal or a polycrystal , but is preferably a single crystal . as seen from a fact that sapphire glass or ruby is used for a bearing of a watch , these gems are hard and have wear resistance , and therefore are hardly worn away even after long - term use for a fountain pen nib . in addition , these gems have chemical resistance , and therefore are hardly corroded by an ink , and use thereof for a nib allows a fountain pen to be used for a long time . in addition , these gems give a luxury feeling , and have not been used as a material of a fountain pen nib conventionally . therefore , these gems can give a luxury feeling and novelty to a fountain pen . for example , use of sapphire glass for the nib makes the nib transparent , and a stereotype that a luxury fountain pen nib has a gold color can be overturned . the shape of a tip of the nib is not particularly limited , but requires to be smooth . for example , the shape of the tip may be a plate shape having a smooth corner . in addition , the tip may be processed into a spherical crown shape , a spherical shape , or the like . by processing the tip of the nib into such a shape , the nib glides on paper smoothly , and a writing feeling is improved . the shape of the nib may be similar to that of a conventional nib . the nib is usually curved inside in an arc shape . in this case , when a curvature radius r thereof is large , the nib repels an aqueous ink , that is , a capillary phenomenon hardly acts , the ink hardly penetrates into a slit of the nib , and the ink may be solidified disadvantageously . therefore , by reducing the r or coating the nib at least with a material having higher hydrophilicity than a main material of the fountain pen nib , for example , with magnesium fluoride used as a non - reflection coating agent in a case of using sapphire glass as the main material , penetration of an ink into a slit of the nib is promoted , and an ink can be prevented from being repelled and solidified . r is preferably 20 mm or less , and more preferably from 5 to 15 mm . the curvature radius may be equal over the nib , or may be slightly larger as closer to the tip . noted that it is not always necessary for the curvature to have a perfect arc shape . coating of the nib can be performed by a known method such as vacuum deposition , sputtering , or cvd . coating may be performed on the entire nib , but may be performed on the entire nib except for a tip of the nib , only on surrounding of a slit of the nib , or only on a surface of the slit of the nib , to which an ink flows from a pen core . the width of the slit of the nib is preferably 0 . 2 mm or less , and more preferably 0 . 1 mm or less . it is possible to write a finer letter with a narrower width . the fountain pen nib of the present invention is manufactured by polishing a gem as a main material . for example , a case where sapphire glass is used as the main material will be described below . from so - called plano - convex cylindrical type sapphire glass having a semi - cylindrical shape , the gem is sliced such that the thickness of the material , the vertical length thereof , and the horizontal length thereof are appropriate to obtain an appropriate size . the gem is sliced so as to have a so - called nib shape which is thinner toward a tip . furthermore , front and back surfaces of the semi - cylindrical shape are sliced such that the nib is curved in an arc shape . a slit is formed in the nib , an outer periphery is subjected to chamfering , brushing is performed , and the outer periphery is polished finally . when sapphire is sliced , each surface thereof is sliced , and an uneven portion is polished to give a luxury feeling . as described above , the nib of sapphire glass , ruby , or the like , which has been subjected to hydrophilic coating has unprecedented durability and wear resistance , and gives a luxury feeling . however , it has been found that the hydrophilic coating is deteriorated by an ink , hydrophilicity is lowered , and discoloration or blurring in letters may occur after long - term use of the nib . therefore , the present inventors made further studies , and have found that hydrophilicity is improved by performing a surface - roughening treatment ( so - called satin finish ) in place of the hydrophilic coating . by a rough surface , water repellency is suppressed , the nib is easily wetted , an ink easily flows to the tip , and blurring no longer occurs . furthermore , the present inventors have found that an ink flows smoothly to the tip even after long - term use , and blurring or the like does not occur by forming a thin groove inside the nib in place of a heart hole and a slit which have been usually formed in a conventional nib and subjecting the groove and surrounding thereof to a surface - roughening treatment by a laser . the groove is formed longitudinally and almost linearly on a backside ( inside an arc ) of the nib . the groove may be formed over the entire length of the nib from a root to a tip , or may be formed from almost the center in a longitudinal direction of the nib ( part in which a heart hole is conventionally formed ) to the tip of the nib . the depth of the groove depends on the thickness of the nib , but is usually from 0 . 1 to 0 . 3 mm , preferably from 0 . 15 to 0 . 25 mm , and particularly preferably about 0 . 2 mmm . the width of the groove on a front surface of the nib is from 0 . 1 to 0 . 3 mm , preferably from 0 . 15 to 0 . 25 mm , and particularly preferably from 0 . 2 to 0 . 22 mm . the cross - sectional shape of the groove is not particularly limited , but is usually a v - shape , a semicircular shape , a semi - elliptical shape , or the like . an inside of the groove and surrounding thereof are preferably formed into fine rough surfaces , that is , satin surfaces by laser processing or the like . in a similar manner to the above , by a rough surface , water repellency is suppressed , the nib is easily wetted , an ink easily flows to the tip , and blurring no longer occurs . the surface - roughening treatment is preferably performed by a laser , but may be performed by another method such as a sandblasting method . in addition , as a collateral effect , an ink hardly oozes on a front side of the nib , and ink contamination does not occur because a heart hole or a slit is not necessary . in addition , by performing a surface - roughening treatment by a laser , retention of an ink is improved , and therefore dripping of the ink from the nib hardly occurs . utilizing the improvement of retention , by adjusting a pressure applied to the ink , the amount of the ink can be adjusted , and the thickness of a letter can be adjusted . a groove is usually formed on a backside of nib with a diamond cutter . a desired cross - sectional shape can be formed according to an edge shape of the cutter . for example , a v - shaped groove can be easily carved with a rotary blade having an acute angle edge . to form a conventional slit , cutting needs to be performed with an extremely thin cutter to cause blade chipping or the like , and therefore an advanced technology is necessary . in contrast , in order to carve a groove , it is not necessary to make the entire cutter thin , and therefore processing is easy . in a case of the slit , an extremely thin nib tip makes its strength weak , there is a limit to the thickness of the nib tip , and it is difficult to manufacture a nib to write an extremely thin letter . however , in a case of the v - groove , the tip is not split , therefore much thinner processing can be performed than in the case of the slit , and the v - groove is also suitable for manufacturing a nib for writing an extremely thin letter . hereinafter , the present invention will be described based on the drawings and examples . fig1 shows a front side of a nib in an example of a fountain pen nib of the present invention using sapphire glass . the sign 1 represents the nib , the sign 2 represents a slit , the sign 3 represents a tip , the sign 4 represents a body of a fountain pen , and the sign 5 represents a heart hole . the nib is curved toward a backside of the drawing . the tip of the nib 3 has a smoothly polished plate shape having an almost semi - circular shape in a top view . surrounding of the slit except for the tip 3 is coated with magnesium fluoride . when a fountain pen was manufactured using this nib according to a usual manner , was subjected to a writing test , and was compared with a conventionally commercially available fountain pen using an 18 - karat gold nib , a so - called writing feeling was equal to each other . the fountain pen of the present invention gave an extremely luxury feeling . fig2 ( a ) to 2 ( c ) show a nib , having a cross section v - shaped groove formed on a backside of the nib in place of the slit and the heart hole in fig1 . fig2 ( a ) is a diagram of the backside . fig2 ( b ) is an arrow cross - sectional diagram cut along a - a ′. fig2 ( c ) is an enlarged diagram of the b portion . a groove 12 having a cross - sectional v shape is linearly formed from a root to a tip on the backside of the nib . a fountain pen was manufactured using this nib according to a usual manner , was subjected to a writing test , and was compared with example 1 . as a result , a writing feeling was equal to example 1 . a durability test was performed using the nib in example 2 . a copy paper was wound around a disk - shaped metal rotation rotor having a diameter of 178 mm . while a nib was pressed against the roller at a contact angle of about 45 degrees , rotation was performed at 3 . 4 rpm , and the wear degree of the nib was observed . after 100 hours , the nib was observed , and worn of the nib was hardly observed . this nib was subjected to a writing test again , and a writing feeling was equal to that at the beginning . a test similar to example 3 was performed using a wood rotation roller while a nib was pressed directly against the roller . the rotation speed was 1 . 7 rpm . after 48 hours , the nib was observed , and worn of the nib was hardly observed . a test similar to example 4 was performed using a metal roller while a nib was pressed directly against the roller . the rotation speed was 1 . 7 rpm . after 24 hours , the nib was observed , and worn of the nib was hardly observed . the fountain pen nib of the present invention is useful particularly as a luxury fountain pen nib .