Until recently, only emerald, ruby and sapphire were officially called precious gems. Everything else was called semi-precious. The precious categorization is a reference to value: a really fine ruby, emerald or sapphire can be priced higher per carat than a diamond. Sapphires are extremely hard and durable, so they will last a forever in a piece of jewelry, and rubies and emeralds are extremely rare. There are some rare finds among the so-called semi-precious gems that can be priced higher than the big three, but in general, a fine ruby, sapphire or emerald will hold its value and command more respect and a higher price than other gemstone. That is the reason Art Deco jewelry – heavy on the big three – is usually prized over other period jewelry.