According to the site Violet crystals can be found here:
Violet Crystal Location(s)
Dantooine (Crystal Cave) - Obtained from a crystal formation
Tatooine (Eastern Dune Sea) - Found in rubble within Krayt Dragon lair
Korriban (Tomb of Tulak Hord) - Found in urn
But never found one on Dantooine
[EDIT]You can do Tatooine or Korriban whenever you want, so there isnt realy one you find earlyer then another.
I've been through several times (just went through once then to confirm, in fact) and there's no violet crystal in the Dantooine cave. There's a red, blue and yellow, ten reds in the kinrath eggs, a bondar and two rubats.
Also wacked all the eggs, and I'm pretty sure I've clicked on all the formations when I went through a few days ago, and ended up with the same crystal combination as previous poster. Could it be a random crystal (not that it seems much is random in the game though )?