go for geode

black opal set in clear glass and rolled in glass layers to create a 'geode' in a white glass ring
sarah lewis jewlry @ www.etsy.com

http://www.jacquieaiche.com/rocksandminerals.html

Dipped Geode Ring$168.00 www.freepeople.com

Geodes are essentially rock cavities with internal crystal formations- all unique in shapes, sizes & colors- so great for one of a kind jewelry that will always be a conversation piece.