The case of the gemstone.

From:  "Wondering Wilson"

Are the citrine and the yellow topaz the same stone? Does the yellow topaz occur in nature, or is it heat altered or chemically altered to produce that color?

Mark Wienants thoughtfully pondered this: 

Strictly speaking, citrine is a yellow variety of quartz (hexagonal silicon dioxide [SiO2]), and topaz is a semiprecious gemstone (rhombic prism fluosilicate of aluminum [Al2SiO4(F,OH)2]). Since they are chemically and structurally different, no, they are not the same stone. Yellow is found in nature--impurities in the stone give it its color--and is generally considered the most precious of the topaz. Citrine, produced by heating smokey quartz, is also called false topaz.

Mario responded with this: 

Yellow Topaz are naturally occurring in nature, much like yellow diamonds, but in both cases because these stones are generally rare in natural settings, they can and are in many cases chemically treated to produce their yellow effect. Citrine is actually a yellow quartz that is used to imitate yellow topaz.