Spinel
Magnesium Aluminum Oxide, MgAl2O4
Crystal System: Cubic
Hardness: 7.7-8.0
Density: 3.5-4.1

Spinel occurs as an accessory mineral in igneous rocks such as gabbro. Spinel is the name of a series rather than a specific mineral. It also occurs in in contact metamorphic rocks, especially marly dolomitic limestones rich in magnesium and aluminum, where it is associated with phlogopite, graphite and chondrodite. Due to it's hardness and resistence to weathering, spinel occurs commonly in alluvial and marine placers.

Spinel crystals occur as transparent to opaque octahedrons which are commonly twinned. It's color is quite variable, ranging through black, red, blue, green, brown, and colorless. Iron, zinc and manganese atoms can substitue for magnesium in its structure, causing variations in color and physical properties.

While magnetite, chromite and ganhite are all members of the spinel group, the characteristic gem spinel excludes these minerals. The group of spinels used as gem stones are transparent and often of attactive color. The most usual colors seen are ruby red and rose red.

Stuart provides the following remarks:

"This is four views of one large floater Spinel octahedron from the ancient locality of Mogok, Burma (currently known as Myanmar). It has very fine deep "ruby" red color and measures 1 inch on each edge."

Rocks from Stuart Wilensky's Collection

Index of Specimen Images

Table of Contents

bkeller@rockhounds.com 7/30/95