How do crystals work theoretically and what types of crystals give best results for what things (meditation, astral travel, warding off negative spirits and energies, summoning, channeling etc)?
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 :cool: .
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.
I've experienced that Citrine is a good crystal for channeling reiki and back off "evil gaze" of others. It has the property that is not able to hold negative energy (cyanite does not either), neither does it need "rest". It may be charged though. It can also be programmed (I do this a lot with clear quartz for various purposes). It does not need any maintenance like many other types of crystals probably do. Citrine is not very expensive. Try to find a clear one though. The clearer, the better, the "smoother" the energy that is "transmitted", "goes through" etc. I cannot really express it. Try to experiment with it. You can use clear quartz for that too.
There is a book by Judy Hull that you can get at any book store or metaphysical store and the book is call the Crystal Encyclopedia. It is relatively cheap only 25 dollars and It has all the common Crystals and rocks and the Rarer ones two. :-D