Amethyst is a transparent purple variety of quartz and is one of the most highly valued stones in the quartz group. Quartz has a strong framework structure that makes it hard, free from cleavage, and therefore very durable. As late as the sixteenth century, amethysts cost as much as diamonds and were equally valued. Due to the stone's beauty and scarcity at the time, it was often set with rubies, sapphires, emeralds, and diamonds in almost every ancient European crown, scepter, and sword. The birthstone for February, the name of the gem comes from a Greek word that means "not drunk." This is due to an ancient belief that an amethyst protected its wearer from inebriation and consequentially made the gem highly desirable by those so inclined.