https://eq.magelo.com/item/25814/Flawless-Diamond
This is magelo's data of where they drop on live. Might be different here.
What were you killing in Velks? I've seen a couple of chest gems drop from stuff in velks close to where velks himself spawns. Also on the dragon in CS. All priest gems though.
Chest gems seem pretty rare and only to drop off the highest mobs. I hear SG is a good place for them in p99. I've not done SG myself though.