Twsbi Diamond 580 Iris Fountain Pen - 3 nib sizesA great fountain pen with an iridescent nib and finish. The nib, cap metal accents, inner mechanism and end cap metal accents are all iridescent. The idea behind the TWSBI Diamond was to go back to basics and capture the simplicity and elegance of traditional writing instruments. It is a classic fountain pen with a piston ink filling system. The Twsbi 580 has an increased ink capacity compared to 530 version about 30% more. It also has improved