Cubic Zirconia

• Cubic zirconia sparkles, but not like a diamond.Ryan McVay/Photodisc/Getty Images
  
  This synthetic stone is a crystalline form of zirconium oxide. The gem shares diamond's high index of refraction, the property that gives diamond and cubic zirconia their fire. However, cubic zirconia is considerably heavier than diamond; a one-carat diamond and a three-carat cubic zirconia stone are about the same size. Cubic zirconia also has more prismatic color than diamond, a giveaway of the stone's real identity. Diamond testing equipment also reveals cubic zirconia as it is a thermal insulator while diamond is a conductor.
  
  

Moissanite

• The newest simulated diamond on the market, moissanite made its way into jewelry stores in 1997. It has a natural counterpart, but natural moissanite is so rare that virtually all available stones are synthetic. Moissanite's hardness is almost as great as that of diamond, as is its thermal conductivity; the stone successfully foiled thermal diamond testing equipment until the advent of newer models that measured electrical conductivity as well. Moissanite's double refraction also sets it apart from diamond.
  
  

Clear Sapphire

• Clear and pale sapphires are comparable to diamonds in clarity, but not in sparkle and fire. Sapphire's lower index of refraction keeps light traveling through the stone rather than bouncing and scattering within it. A sapphire turned on its table -- the large facet at the gem's top -- is clear enough to allow a viewer to read text through it. A diamond's high refractive index won't permit light from the printed page to travel through it.
  
  

Glass

• Glass baubles provide sparkle at a minimal price.Creatas/Creatas/Getty Images
  
  The least expensive diamond alternative is also the oldest. Glass poured into diamond-shaped molds or melted into facets superficially resembles the shape of a diamond, but lacks the precious gem's fire. Faceted glass scratches easily and forms soft edges between facets. While they look little like genuine diamond, glass rhinestones add a great deal of sparkle for a small price, making them an affordable alternative for costume jewelry or clothing trim.