Uni-Ball Signo Extra Fine Point Gel Pen Refills, Black Ink, 0.38 mm, Set of 6 (Japan Import) [Komainu-Dou Original Package] Refill for Uni-Ball Signo 0.38mm Extra Fine Point Gel Ink Pen UM151.24 Ink color: Black The diameter of a ball: 0.38mm. Extra Fine Point.