Is the mineral alexandrite precious or semiprecious, and what is its number on the Mohs scale.?

Answer 1

Alexandrite is a variety of chrysoberyl, a semi-precious stone. It's hardness is 8.5 on the Moh's scale. See:
http://www.gia.edu/library/28235/featured_gemstone__alexandrite.cfm

Answer 2

I do hate the term "semi-precious" Yes, chrysoberyl is not among the big five precious stones but I have in my stock alexandrites costing more than $20,000 per carat. Hardly SEMI precious I think.

Answer 3

The hardness (Mohs scale) is 8.5. It is a rare mineral found in the Soviet Union in the 1800's.It is a semi-precious mineral.
Below are some other characteristics of Alexandrite.

Alexandrite Chemical composition: BeAl2O4 (small scale replacement of alumina by chromic oxide causes alexandrite's characteristic colors)
Class: oxides
Crystal system: orthorombic; 2/m2/m2/m
Crystal habit: prismatic crystals are elongated on c, and often tabular on {001}, with striations parallel to [100]
Twinning: commonly twinned on {130} resulting in a triangular or kite shaped form, repeated cyclic twinning produces pseudohexagonal "trillings"
Specific gravity: 3.68 - 3.75
Index of refraction: 1.745 - 1.759
Birefringence: weak (0.008-0.011)
Pleochroism: strong, X= red-purple, Y= orange, Z= green
Hardness: 8.5
Color: green in daylight and red-violet in incandescent light.
Luster: vitreous to greasy
Transparency: transparent
Cleavage: distinct on {011} perfect, indistinct on {010}, and poor on {100}
Fracture: conchoidal, brittle
Streak: white