IMO crystals work a) through setting off assocations, probably deep and at the collective subconscious level (due to shape, colour, beauty, etc), and b) due to the crystalline structure adding shape power on a microscropic to macroscopic level and c) through their resonant properties (eg quartz, think watches) and there is probably a d) which I dont know about, maybe e)'s f)'s etc

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As for whats best for what, there are so many crystals out there, I couldn't possibly hope to answer that in this post. There are good books listing it all tho, and a lot of good websites out there.