I don't know the answers to all of your questions, but pertaining to the different stones and crystals and their purposes, here's an article that was already posted on here with a list of some different crystals and their attributes. Of course there are like way more crystals to explore but there's a good bit on there :)
Crystal magic is won of those things that works because you believe in it. If you will your energy to charge a certain stone, and perform the appropriate energy manipulation, it will. Some people put charged amathyst under thei pillows to improve psychism. But it works just as well with any old rock if you believe it. However, those semi precious stones make excellent offerings and altar decorations.