Gems are random in value, within a specific range. However, when cutting them the value of the final gem is rather predictable.
There is a very helpful guide to gem value after cutting in the "The Sims 3 Guide". Click on the link above, select Skills from the menu, then collecting, then Gems.
Actually, almost everything you might want to know about collecting is covered in the artical indicated above.
Collecting everything should keep you busy for quite some time. Breaking space rocks, after you have mastered martial arts can yield a lot of gems. All sorts of collectables can be optained with the miner (available from inventing or occassionally found for sale in the consignment store). I find this is the best way to find all the space rocks, although, the finds from mining are random.)
Gems, metals and relics to complete the relic collections can be found in the relic store in the market, though the availability is random with rare relics turning up only very occassionally.