Oxides and Hydroxides: Spinel

Composition: MgAl₂O₄; iron, zinc and manganese may substitute for magnesium

Crystal system: Isometric; crystals usually octahedral, also massive granular

4.12.1
ruby spinel
Sri Lanka

Properties:

• Vitreous lustre
• Various colors including almost colorless, red, green, blue and black
• White to grey, green or brown streak
• Knife does not scratch (Mohs Hardness 7.5-8)
• Above average density, increasing with substitution of iron, zinc and manganese
• No cleavage
• Conchoidal fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)
  

Occurrence:

Spinel occurs as an accessory mineral in igneous and metamorphic rocks and in heavy mineral sand and gravel deposits.

4.12.2
ruby spinel crystals
Sri Lanka

Gem quality spinel occurs in a variety of colors; the red and rose-red varieties are most valued.