Opal

Opal is a pretty common ore that often spawns on the walls and near the surface of the mines. Its appearance is a few crystals with varying colors, along with a larger one in the middle. When mined, it drops 900xp. It is used for making an Opal Pickaxe. (If anyone knows any other uses, please put them here!)

Opal in real life
It is a fairly white rock material, made from tiny spheres of silica and lots of water.