We're actually extremely capable of making gemstone-quality synthetic minerals of many kinds, to large sizes, and to impressive clarities. In fact, synthetic rubies and sapphires are on average better-quality than natural ones. Synthetic emeralds are almost universally higher quality than natural ones, because they can be carefully grown without inclusions, while natural emeralds are almost always heavily included. Synthetic diamonds can be enormous, crystal-clear, and are dirt cheap on the production side of things.
Gemstones as an industry is something controlled by marketing. The desire for a rare natural stone almost universally outweighs people's desire for jewelery for its own sake and that's something that's heavily controlled by a grip of marketing. You could go buy a 15 carat crystal clear pigeon's blood ruby for $25 right now, if you wanted.