Value is determined by supply and demand. Supply is determined by the scarcity of the stone. Demand is controlled by the desirability of the stone. People long ago decided that certain stones were more desirable because some of them were more pleasing to look at. Rubies, sapphires, diamonds, and emeralds are among the most valuable because (gem quality stones) are quite pleasing to look upon and they are among the rarest of the gemstones. However, as ideas of fashion change, the values of these stones can change. There is one thing that should be noted here. The price of diamonds is artificially high. Diamond production in the world is almost completely controlled by the De Bears cartel. Since they control most of the diamond production, they can and do limit the supply of diamonds. When people have found other deposits of diamonds, if they don't get into bed with De Bears, then De Bears floods the market with enough diamonds to drive the price down until the opposition is forced out of business. I once had hopes that Russia would break their monopoly. (Russia has rich diamond deposits of its own and the Soviet Union accumulated stockpiles big enough to actually challenge De Bears.) Unfortunately, Russia just struck a deal with De Bears and the status quo has been maintained.