Cobaltite is a brilliant blue stone (notably, the only stone of its color) found as veins in numerous environments. Despite its appearance (showing up as £ unmined and * mined), it is not a usable ore.
Cobaltite is particularly dense, making it useful for stone-fall traps and as catapult ammunition.
In Real Life
Cobaltite is an ore of cobalt, a metal not modelled in Dwarf Fortress. Interestingly, the name derives from the word kobold.