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.

The name ceruleite comes from the Latin caeruleus, meaning “sky-blue”. Many minerals are named for people or places—ceruleite is named purely for its color.

Its color is so intensely pure that gemologists sometimes compare it to high-grade turquoise (though it’s a different chemistry entirely). Collectors sometimes prize it as a “super rare turquoise lookalike.”

Antique collectors loved it, but gemmologists banned it

Because ceruleite rarely forms crystals, old collectors would cab or polish pieces for Victorian jewelry, but it contains arsenic, it is too soft for long-term jewelry wear, it is heat-sensitive, and is somewhat brittle so ceruleite cabochons are no longer cut, making antique pieces extremely collectible.