I think Compendium has a good point to let the crystal choose you. Personally I wouldn't get too hung up on where it was mined. IMHO if a crystal has the same chemical formula it shouldn't matter too much which soil it was taken from. After all, the techtonic plates are always shifting anyhow.
As far as power is concerned, I would think that bigger is better, although many would not agree with me. If it were me, I would get the prettiest, most impressive ones, something you will get a lot of joy from.
