If you are lev 61+ the regular gem drop quality are Marquise gems. If you play difficulty Master or higher you also have a chance that a gem of Imperial quality drop. (see here)
Benefits of gems
With the jeweler you can upgrade your gems. You need 3 gems of same quality and sort, gold and for the Royal and Flawless Royal gems you also need 1 Death Breath's, but I will ignore these, because if you are allready upgrading your gems you should have enough Death Breath's.
Through leveling your Jeweler you learn the recipes to upgrade to Imperial and Flawless Imperial gems. For Royal and Flawless Royal gems you must find the design.
How much cost a Flawless Royal gem?
1 Flawless Royal gem cost 81 Marquise gems and 9,8 million gold .
1 Flawless Royal gem cost 27 Imperial gems and 4,4 million gold.