Any rare crystal that is created through an artificial process. Synthetic gems are usually made in "bombs" which are either stainless steel or platinum. They are made to withstand high temperatures (2000+ C) and pressures over 100 atmospheres. The trick is controling the temperature and inclusions during the cooling process. Most synthetic gemstones display phosphoresence after being exposed to UV light, due to included nitrogen (wich diffuses out during the thousands/millions of years during which natural gems are formed). Artificial gems also have boundaries where it can be seen where the seed crystal was and the new growth began.

The stuff gems are made of...

• diamonds - carbon
• goshenite, morganite, aquamarine, golden beryl, emerald - beryl
• ruby, sapphire - corundum
• opal - amorphous silica
• jade - jadeote (a pyroxene)
• cats' eye, alexandrite - chrysoberyl
• topaz - a silicate

Swiped mercilessly from an email written by an old roommate (so no, this isn't copyrighted material)

Note: "synthetic gem" specifically means it's a lab-grown stone of the same structure as the real gem it's imitating. In the jewelry trade, since something like a cubic zirconia is not imitating any particular stone, it is called a "simulant" or "simulated" (for example, a red CZ would be "simulated ruby," not "synthetic ruby.")