Force Sensitive Archive

Some mobs like Tuskens and Nightsisters will drop the clear damage crystals. Other types of mobs like marooned pirates drop the color crystals.

As far as the listed quality of the crystal, you're more likely to get a higher quality crystal off of a tougher mob like a Nightsister Elder than one of the lower Nightsisters.

For various mobs and crystal types, here's an older post that may help. (Then again, this was back during the time of the revamp, so the list may have been changed)

http://forums.station.sony.com/swg/board/message?board.id=jedi&message.id=391124

As far as the twin force crystals go, they don't drop anymore, but during the revamp, all thetwin force crystals were converted to damage crystals. If it's clear and hasn't been tuned already, you should be able to use it.