Что такое «муассанит» и чем он отличается от фианита и синтетического алмаза?

There are two concepts "diamond imitation" and "synthetic diamond" which are often confused.
Imitation is a material (in our case a mineral) that has an external resemblance to the object of imitation but differs from it in composition and physical properties. Unlike simulation the synthetic analog of the material is similar to the natural analog of the chemical composition (without taking into account impurities obtained during the synthesis) and properties.
Moissanite and phianite are the most common in the market of precious stones of imitation of a diamond, having a synthetic origin.
Moissanite is a mineral of SiC (silicon carbide) hexagonal system, rare on Earth and widely distributed in space. At the turn of the 1990's and 2000's, artificially grown moissanite found application in the jewelry industry as an independent jewelry material and as an imitation diamond. The external similarity of moissanite and diamond is due to similar values of these minerals. You can distinguish imitation by the noticeable birefringence properties of all anisotropic stones, and by specific tubular microinclusions.
Phianite is an artificial material that does not have complete analogs in nature. The composition and crystallographic structure is zirconium dioxide ZrO2 with an artificially stabilized cubic crystal system. The main diagnostic feature of phianite is its very low thermal conductivity in comparison with a diamond. The proximity of the refractive index of the diamond and its imitations ensures their external similarity however differences in hardness and density make it possible to easily distinguish "not a diamond".
Synthetic diamond is a mineral with a chemical composition corresponding to the native carbon of the cubic system and outwardly completely analogous to the natural, differing only in origin. In this connection, it is possible to distinguish a synthetic diamond from imitation in exactly the same way as a natural one.