Robert Oster Blue Velvet Storm (shimmer) fountain pen ink in a 3 mL sample vial.
Shimmer inks are great fun but require a bit of care. Before filling, shake the bottle ever so gently (with the cap on). Prior to writing, gently rock, roll, or twirl your pen in your hand (see: Iceman, Top Gun, 1986) to mix the particles that may have settled. Exercise good pen hygiene, use the pen often, and you will have a good time. Leave it sitting for too long, and you might have a clogged feed.