Crystals have always had a "magical" quality. The ancients would look for crystals that have (as always) been formed by the Earth itself. The ancients would use "coloured stones" when they could not find a crystal; some still do! But the "old ones" always tried to find a "crystal". Now they are "mined". At one time, Amethyst was as expensive as diamonds. Now it is mined, and so much cheaper. I like my crystals to be "natural". I really don't care for them to be carved and "polished" into "jewels".
There are many books on the subject.