The Federal Trade Commission’s Guide for the Jewelry, Precious Metals and Pewter Industries

FTC writes:

23.23 Misuse of the words "ruby," "sapphire," "emerald," "topaz," "stone," "birthstone,"
"gemstone," etc.

(a) It is unfair or deceptive to use the unqualified words ruby, sapphire, emerald, topaz, or the name of any other precious or semi-precious stone to describe any product that is not in fact a natural stone of the type described.

(b) It is unfair or deceptive to use the word ruby, sapphire, emerald, topaz, or the name of any otherprecious or semi-precious stone, or the word stone, birthstone, gemstone,' or similar term to describe a laboratory grown, laboratory-created, [manufacturer name]-created, synthetic, imitation, or simulated stone, unless such word or name is immediately preceded with equal conspicuousness by the word laboratory-grown, laboratory-created, [manufacturer name]-created, synthetic, or by the word imitation or simulated, so as to disclose clearly the nature of the product and the fact it is not a natural gemstone.

Note to paragraph (b): The use of the word "faux" to describe a laboratory-created or imitation stone is not an adequate disclosure that the stone is not natural.

(c) It is unfair or deceptive to use the word "laboratory-grown," "laboratory-created," "[manufacturer name]-created," or "synthetic" with the name of any natural stone to describe any industry product unless such industry product has essentially the same optical, physical, and chemical properties as the stone named.

More info @ http://www.ftc.gov/bcp/guides/jewel-gd.htm