ZOOM/Zoom Refills for C1 Series... The core in the C1 is an ultra-low viscosous oil; but you can also use the gel ink refills for L1. Lead diameter: 0.03 in (0.7 mm) Ink Color: Black Size: Height 5.5 inches (140 mm), Width 1.3 inches (32 mm), Thickness 0.08 inches (2 mm). Product Color Trend:Black