Diamonds, Cubic Zirconia (CZ) and Moissanite are crystalline structures. Diamond is comprised entirely of carbon, CZ is zirconium oxide, and Moissanite is silicon carbide (silicon + carbon). Their crystalline structures are formed under intense heat, in the ground or in the laboratory.

Diamonds and Moissanite occur naturally although Moissanite only in extremely small quantities, and then only in certain meteor craters. All three are very close optically and only a trained gemologist, often with the need of special equipment, could differentiate a diamond from a CZ or Moissanite gemstone.

Diamonds are the hardest substance with a Mohs rating of 10, the highest available number. Moissanite is very close to diamond hardness at 9.25. CZ is also a very hard substance with a Mohs rating of 8.5 - 9.0. All of them will cut glass. Hardness is important for cutting gemstone facets as it determines the intricacy and sharpness of the cuts that can be made. A gemstone's facets also determine how it it will display its fire and brilliance.

A gemstone's fire is its ability to disperse white light into many colors, like a prism. This characteristic is easily noticeable on earrings and necklaces. Moissanite is the fire champ by far and will shoot blue and green flashes in even moderate light. Diamond, the gemstone most historically commented for its fire, has less fire than CZ. It is somewhat ironic that after centuries of valuing diamonds for their fire that Moissanite and CZ get devalued for exhibiting even greater fire.

Brilliance is a gemstone's ability to capture light. When light hits the stone at a certain angle it actually gets trapped temporarily inside the stone. A diamond or other gemstone displays brilliance as the trapped light reflects upon itself and makes the gemstone sparkle in brilliant light pulses. Again the champ, Moissanite has the most brilliance. On the refractive scale CZ is 2.2, diamond 2.4 and Moissanite 2.7. Crown glass, for example, has a refractive index of 1.5.

Diamonds, Cubic Zirconia, and Moissanite look very much alike. They differ in the physical characteristics that matter most for jewelry only by degrees of hardness, fire, and brilliance. A well cut Cubic Zirconia gemstone in an attractive setting would almost certainly be more appealing to the eye than a mediocre diamond piece.

Price and lore are the most conspicuous differences.