In the corundum family, all corundum gems except ruby are called sapphires, so the hardness is also 9. Sapphire is one of the hardest substances on the earth at present, and its molar hardness is 9 (the highest molar hardness is 10), which is second only to the diamond hardness (10). That is to say, only substances with the same hardness as diamonds can "harm" sapphire.

Sapphire is composed of alumina, which is blue because it contains trace elements of titanium (Ti4+) or iron (Fe2+). It belongs to triangular crystal system, and its crystal morphology is often columnar, short columnar and plate-shaped, and its geometric shape is mostly granular or dense block.

Transparent to translucent, glass luster. Refractive index 1.76- 1.77, birefringence of 0.008, strong dichroism. Not an isotropic body. Sometimes it has a special optical effect-starlight effect. The hardness is 9 and the density is 3.95-4. 1g/cm3. No cleavage, cleavage development.

Characteristics of sapphire:

Sapphire is characterized by uneven color, and parallel hexagonal columns are arranged with flat bands and growth lines of different shades. Double crystals are developed in multilayer plates, which are usually louver-shaped, and cleavage often splits along the double crystal plane. Regarding the collection value of sapphire, the quality of sapphire is the key, which can be compared mainly from the aspects of color, clarity, weight and cutting.

Kashmir sapphire is a kind of high-quality sapphire, which is called cornflower blue by the jewelry industry. It is bright and pleasing to the eye because it contains tiny dust-like inclusions, which scatter light and form a velvet luster.