Spinel Crystals

Deep Red Crystals of Spinel Set Atop White Marble Matrix! From the Morogoro, Uluguru Mts (Uruguru Mts), Morogoro Region, Tanzania. Photo: The Mineral Gallery, Inc.

Deep red crystals of Spinel set atop white marble matrix!

Spinel is a very attractive and historically important gemstone mineral. Its typical red color, although pinker, rivals the color of ruby.

Spinel and ruby are chemically similar. Spinel is magnesium aluminum oxide and ruby is aluminum oxide. This is probably why the two are similar in a few properties. Not suprisingly, the red coloring agent in both gems is the same element, chromium. Spinel and Ruby also have similar luster (refractive index), density, and hardness. Although ruby is considerably harder (9) than spinel, spinel's hardness (7.5 - 8) still makes it one of the hardest minerals in nature.

Spinel may be the poorer cousin of ruby, but its pinker color and other qualities make it attractive in its own right. Spinel typically forms nicely proportioned octahedrons. But it is famous for a type of twinning that bears its name, the Spinel Twin Law. Spinel Law twinning is also found in other isometric minerals such as diamond, galena, cristobalite, magnetite, franklinite and other members of the spinel group. This type of twinning produces a twin plane that is parallel to one of the octahedral faces. The plane acts as a mirror plane and produces a left and right side that are mirror images of each other. This may not sound all that spectacular for a very symmetrical mineral like spinel which is loaded with mirror planes. However this mirror plane is not parallel to any of the others and actually lowers the symmetry of the crystal (only in appearances though).

 

Spinel Properties

Chemical formula: MgAl₂O₄

Color is red, green, blue, purple, brown, and black; but red is by far the more common color.
Luster is vitreous.
Transparency: Crystals are transparent to translucent and sometimes nearly opaque.
Crystal System: Isometric; 4/m bar 3 2/m
Crystal Habits include the typical octahedron, but can be found as dodecahedrons and combinations of other isometric forms. Also as rounded grains in alluvial placer deposits.
Cleavage: None
Fracture is conchoidal.
Hardness is 7.5-8.0
Specific Gravity is 3.6-4.0
Streak is white.
Other Characteristics: index of refraction is approximately 1.71 - 1.76 and rutile inclusions may produce six or four rayed stars or asterisms.
Associated Minerals include calcite, dolomite, corundum and garnets.
Notable Occurrences include Burma, Sri Lanka, Brazil and Afghanistan.
Best Field Indicators are twinned crystals if present, color, hardness, density and locality.