Synthetic sapphire has a crystalline and not an amorphous structure. The often used designation as "sapphire glass" is therefore misleading and physically incorrect. It is the single crystalline form of corundum, also known as alpha-alumina, alumina or single crystalline Al2O3.

In each case, it is produced by melting the raw material (alumina) and then cooling and solidifying it. The result is an anisotropic, hexagonal-rombohedral single crystal with 3 axes and planes. Although for most applications a random orientation is sufficient for further processing, it must be remembered that the exact behavior of optical components made of sapphire depends strongly on the crystallographic orientation relative to the optical axis (c-axis, 0001).

With a Mohs hardness of 9, sapphire is the fourth hardest of all transparent materials after moissanite, cubic silicon nitride and diamond, and is therefore particularly valued in applications requiring high scratch resistance. In addition, sapphire's melting point of 2050 �C, physical strength, impact and corrosion resistance, inertness, extreme compressive strength, and unusual combination of excellent thermal conductivity and high heat resistance further set it apart from advanced ceramics and glass. Likewise, its optical properties are exceptional: it is transparent at light wavelengths between