A diamond simulant is a material that looks like a diamond but lacks its chemical composition or crystal structure. This is distinct from a synthetic diamond, which is a man-made stone composed of crystallized carbon atoms, identical in physical and chemical properties to a natural diamond. The most common material used to imitate a diamond is Cubic Zirconia (CZ). The primary metallic element in this simulant is Zirconium, which is combined with oxygen to create the final compound.
The Primary Simulant: Cubic Zirconia
Cubic Zirconia is a laboratory-created crystalline material composed of Zirconium Dioxide. Zirconium forms the structural basis of the compound, but combining it with oxygen yields the specific properties required for a diamond imitation. While Zirconium Dioxide naturally occurs as the mineral baddeleyite, that form has a monoclinic crystal structure unsuitable for a gemstone.
CZ is chosen as a simulant because its high dispersion and refractive index give it fire and brilliance similar to a diamond. The refractive index of CZ is around 2.15 to 2.18, which is slightly lower than a diamond’s 2.42, but still quite high. Its dispersion, the ability to split white light into spectral colors, is high at 0.058 to 0.066, exceeding a diamond’s 0.044. This high dispersion creates the noticeable “rainbow” sparkle associated with CZ.
The material possesses a high degree of hardness, ranking between 8 and 8.5 on the Mohs scale, making it durable for jewelry. Although Zirconium is the identifying element, the final product is a stabilized oxide compound. CZ is a dense material, with a specific gravity ranging from 5.6 to 6.0 g/\(\text{cm}^3\). This makes it approximately 1.65 times heavier than a diamond of the same size.
Manufacturing the Material
The production of gem-quality Cubic Zirconia requires extreme conditions and a specialized technique known as skull melting. Zirconium dioxide has an exceptionally high melting point, reaching approximately \(2750^{\circ} \text{C}\). This temperature is too high for conventional crucibles, so the skull melting process uses the raw material itself to form its own container.
The process involves placing Zirconium Dioxide powder and a stabilizing agent, such as Yttrium Oxide or Calcium Oxide, inside a water-cooled copper container. Stabilizers are necessary because Zirconium Dioxide naturally solidifies into a monoclinic crystal structure, but the cubic structure is required to mimic a diamond. Radio-frequency (RF) induction heating is then used to melt the core of the powder mixture.
The water-cooled container keeps the outer layer of the material cool, forming a solid shell or “skull” that contains the molten material. This internal melting prevents contamination from a traditional crucible. As the molten core slowly cools, large, cubic crystals of Zirconium Dioxide grow, which are then cut and polished into gemstones.
Other Popular Diamond Simulants
Beyond Cubic Zirconia, other materials imitate the appearance of a diamond, each with a distinct chemical makeup. Moissanite, a naturally occurring mineral almost entirely synthesized for jewelry, is a form of Silicon Carbide. Its primary elements are Silicon and Carbon, making its composition significantly different from CZ. Moissanite is known for its exceptional brilliance and fire due to a high refractive index of 2.65 to 2.69, which is even higher than diamond.
Another historical diamond simulant is Yttrium Aluminum Garnet (YAG). Its main elemental components are Yttrium and Aluminum, and it was used extensively before the commercialization of Cubic Zirconia. YAG has largely been replaced because its optical properties, such as dispersion, are not as convincing as CZ. Gadolinium Gallium Garnet (GGG), featuring Gadolinium and Gallium, was also used but is now rarely seen in the market.
Key Differences From Natural Diamonds
One practical method to distinguish a simulant from a genuine diamond is by testing its thermal conductivity. Diamonds are exceptional heat conductors, a property measured by diamond testing devices. Cubic Zirconia is a poor heat conductor and will not register as a diamond on a standard thermal tester. Moissanite, however, is a strong heat conductor and often tests positive as a diamond on older thermal probes.
Differences in weight and hardness are significant indicators for identification. Cubic Zirconia is considerably denser than diamond, meaning a CZ stone will feel heavier than a diamond of the same dimensions. On the Mohs hardness scale, a diamond rates 10, whereas CZ ranks at 8 to 8.5. This makes CZ more susceptible to surface scratching over time. Moissanite is closer to diamond in hardness, registering at 9.25 on the scale.
A gemologist can easily spot Moissanite because it exhibits double refraction, an optical property where light entering the stone splits into two rays. This causes a noticeable doubling of the facet edges when viewed under magnification, which is absent in a singly refractive diamond or Cubic Zirconia. The extreme fire of both CZ and Moissanite is visually distinct, often producing a colorful flash that is more pronounced than the subtle brilliance of a natural diamond.