Simulated Diamonds
Diamond simulants (also known as simulated diamonds) are substances that look similar to a diamond but do not have the same properties (weight, specific gravity, refractive index, hardness, etc.) and are not a carbon compound with the diamond structure. These would include Moissanite, Cubic Zirconia (Van Gaff, Russian Brilliants, Asha®, Diamonique, Diamonelle and Zirconite), White Sapphire, YAG (Yttrium Aluminum Garnet), Rhinestones (glass), to name a few.

Cubic Zirconia
Cubic Zirconia (CZ) is zirconium oxide (ZrO2), a mineral that is extremely rare in nature but is widely created in a laboratory for use as a diamond simulant. The lab created material is hard, optically flawless and usually colorless. It can also be made in a variety of different colors by adding oxides of various elements.
Because of its low cost, durability, and close visual likeness to diamond, synthetic cubic zirconia has remained the most gemologically and economically important diamond simulant since 1976. Even though the popular Cubic Zirconia has more fire (but less brilliance) than a diamond, it is definitely not "forever." A CZ is softer than a diamond, so it does not take as fine of a polish and its brilliance will fade over time. Cubic Zirconia is about 75% heavier than diamond so a CZ the same size of a one carat diamond weighs about 1.75 carats.
There are many variations of cubic zirconia, each claiming to be a better diamond simulant. Most cubic zirconia have a problem with heat from the jewelry repair torch when the owner wants to repair or re-size the ring without removing the stone. The Van Graff is an Yttrium Oxide stabilized Cubic Zirconia with a rare earth dopant formula that makes this material resistant to the flame of a torch without breaking or changing color.

Asha Diamond Simulant
Asha® is unique among the myriad of generic diamond simulants because it the only simulant that employs true lab created amorphous diamond (pure carbon) in its construction. In simple terms, it is the diamond world's equivalent of the cultured pearl. Thus, Asha is created by utilizing a hand-cut non-precious core, and then coating/infusing the entire outer surface with lab-created pure carbon amorphous diamond (~85% diamond bonds). For ease of understanding, Asha's structure is very comparable to how cultured pearls are made except this is being done for simulated diamond. By comparison, cultured pearls use a polished shell bead which is then coated with pearl nacre to produce the most convincing pearl available.
Just like cultured pearls offer you the beauty of pearls at a fraction of the natural pearl price, Asha® is able to offer you the same brilliance and beauty of diamond, but at a fraction of the natural diamond price. Asha sells for about $150 to $197 per carat. The current version of Asha&Rreg; is similar in hardness to sapphire, or 9.0 on the Mohs scale.

Russian Brilliants
Russian Brilliants® is a laboratory grown diamond simulant. It was originally developed for use in the Russian satellite space optics program. These cultivated gems are hand-cut and come in different shapes and sizes. With their brilliance and fire they possess many of the same characteristics and optical properties of a natural diamond. They are very hard and will cut glass!

White Sapphire
White Sapphire, along with its sister the Ruby, is of the mineral corundum. While colored sapphires owe their colors to varying chemical combinations, white sapphires are pure corundum and therefore colorless. With a hardness of 9 (corundum is the hardest mineral known next to diamond), white sapphires are very durable, making them an excellent choice for any jewelry piece.

YAG
Yttrium aluminum garnet (YAG) is a man-made material used in lasers that duplicates the atomic structure of garnet. Colorless YAG is used as a simulant for diamond but is not as common now because it has been replaced in popularity by Cubic Zirconia as a simulated diamond.

Rhinestones
Rhinestones are artificial clear or colored stones of high luster made of glass, paste or gem quartz that are intended to simulate diamonds. The best rhinestones are cut from quartz crystal. Natural crystals from the bottom of the river Rhine in Germany were early substitutes for diamonds; hence the term Rhinestone. Some people also refer to them as crystals or stones. Generally, the stone's cut combined with the overall quality determine the price. The higher the grade, the more the rhinestone will sparkle.