I have a soft spot for gems, but where it comes to pricing them for my world, they are a bitch. There are no statistics for what quantity of gems are unearthed from the ground (I've looked) with the exception of diamonds ... in which case the amount of industrial quality vs. gem quality is never specified. There are reasons for this, no doubt - the industry doesn't want us to know how common these gems are, I would guess. At any rate, the substantial cost of a gem is not in its location. As anyone knows who has bought a rock tumbler, the cost of gems is in the finishing.
Agate, quality: America, China, India, Iran, Madagascar, Mexico, Oregon, Rio Grande do Sul, Salm County (Germany), Uruguay.
Cairngorm: Aberdeenshire, Scotland.
Cat's Eye: Brazil, Ceylon, China, India.
Hawk's Eye: South Africa.
Lapis Lazuli: Afghanistan, Badakhshan, Lake Baykal (Siberia), Chile, Coquimba Province (Chile), Hindu Kush (Pakistan).
Malachite: Arizona, Australia, Chile, South Africa, Zaire, Zimbabwe.
Mother-of-Pearl: Arafura Sea (Australia-New Guinea), California, Dahlak Islands (Red Sea), Saudi Arabia.
Prase: Erzgebirge (Germany), Finland, Salzburg (Austria), Scotland.
Quartz, quality: Afghanistan, Arkansas, Derbent, Eritrea, Grampian Hills (Scotland), Loire-Inferieure (France), Mayo County (Ireland), Orel (Russia), Orne (France), Saudi Arabia, Sinkaing (China), Vishniy-Volochek (Russia).
Quartz, rose: Brazil, Madagascar.
Rhodochrosite: Argentina, Colorado, San Luis (Argentina).
Tiger Eye: Burma, California, India, South Africa, Western Australia.