|MadSci Network: Chemistry|
In short: Chemical impurities in a crystal can produce many many colors.
Since "citrine" is the variety of quartz that is the most gem-like the
trick would be to find citrine that happens to be blue. So on the
internet at your favorite search engine you might search on "blue
citrine"; I found an interesting result on eBay:
Try the links in the MadSci Library for more information on Chemistry.