From Siobhan Montague
Mostly marketing! According to the German expert Max Bauer, precious stones are “the minerals which combine the highest degrees of beauty, hardness, durability and rarity”. These include diamond, ruby, sapphire and emerald. However, it is hard to distinguish between precious and semi-precious stones. Amethyst is only classified as semi-precious because it is more common than opal or turquoise, even though it is harder. In practice most ‘precious’ stones have received this label down to economics and aesthetics, rather than any mineral or chemical properties.
Answered by Robert Colvile for Brain Dump, How It Works issue 85