Ceruleite is a rare, semi-precious gemstone with a range of blue colors, including cerulean blue. It's a hydrated copper aluminum arsenate, and its name comes from the Latin word caeruleus, which means "sky blue".

Beyond rarity and expense, however, ceruleites pose additional challenges for jewelry enthusiasts. These gemstones have a lower hardness (5-6) than more common jewelry stones such as quartz.