Alabaster

Its a common crystal. I usually go a place with many spawners, like the Oasis Springs park. Or, have the collecting sim join the scientist career. There are so many spawners at the lab, its ridiculous. You can pretty much finish collections there, maybe except a few very rare items. Also, you can check the phone/computer if you can actually buy the crystals too. Not sure if that option came with GF and the Crystal Crown, though (that they added crystals as a gift buying option).