Amethyst and citrine are actually two types of quartz. In both cases, the Silicon dioxide crystal lattice of the rock crystal contains impurities of iron, which impart the color to the stone. Amethyst, of course, is purplish, while Citrine is generally, yellow, or brownish. Ametrine is a single rock crystal containing zones colored as both amethyst, and citrine.
Commercial citrine jewelery is often produced by heat-treating amethyst. The heat causes the iron to form different bonds, and the stone changes from a purple color to a yellow-brown color. Ametrine is also currently being produced through differential heat treatment, but generally, the result has the amethyst coloration on one side, and the citrine coloration on the other.
Now gemstones are generally intended to be very personal gifts. Therefore, I propose that we use sets of converging lasers to heat specific portions of the original amethyst crystal, thereby making it possible to monogram a gem in three dimensions.-- ye_river_xiv, Jan 04 2012 Ametrine http://en.wikipedia.org/wiki/Ametrinepurple and yellowish gemstone. [ye_river_xiv, Jan 04 2012] 3D data storage ...-- 8th of 7, Jan 07 2012 Storage, yes, but retrieval?-- mouseposture, Jan 07 2012 Aaaaaach, always with the petty details already !-- 8th of 7, Jan 07 2012 [+] ooh, pretty.-- FlyingToaster, Jan 07 2012 random, halfbakery