Selenite
Selenite takes its name from Selene, the Greek moon, for the pearly sheen along its cleavage planes. It is a variety of gypsum, soft enough to scratch with a fingernail, and it will slowly dissolve in water, which surprises people who buy it for a bathroom shelf.
Enormous crystals form in the right conditions. Mexico's Cave of the Crystals holds selenite beams over ten metres long, grown in mineral-saturated water heated by magma below.
Its softness meant it was rarely used for tools or jewellery, so it comes to modern practice with very little historical baggage attached.