Sunstone, whether in rondelles or briolettes, is shimmering with sparkles typical of feldspars. It makes the gemstone look like millions of little stars sparkling through it. This effect is called "schiller" and may be very sparse or, in finer stones, very dense. Sunstone may range from almost colorless to yellow, orange or red, and in extremely rare cases, other colors. Much of the color and shimmer in most sunstone results from thousands of extremely tiny bits of copper. As light moves over it, the bits positively seem to glow! 

http://www.oregongeology.org/sub/learnmore/sun