Oxides · Sourced PROFILE
Spinel
MgAl₂O₄
A durable oxide mineral whose transparent red varieties have long been confused with ruby.
AT A GLANCE
- Formula
- MgAl₂O₄
- Crystal system
- Cubic
- Space group
- Fd̅3m
- Mohs hardness
- 7.5–8
- Density
- 3.6–4.1 g/cm³; composition controls the range
- Optical character
- Isotropic
CRYSTALLOGRAPHY
Crystal form & structure
Octahedral crystals are classic; also dodecahedral, granular, or massive. Contact twins on {111} define the spinel law.
HAND-SAMPLE PROPERTIES
Physical properties
- Cleavage
- Imperfect parting on {111}; true cleavage is indistinct
- Fracture
- Conchoidal to uneven or splintery
- Tenacity
- Brittle
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to opaque
- Observed colours
- Colourless, red, pink, blue, green, violet, brown, or black
LIGHT & OPTICS
Optical properties
- Optical class
- Isotropic
- Refractive index
- n ≈ 1.719 for Mg-rich spinel; rises with Fe, Zn, and Cr substitution
- Fluorescence & electrical behaviour
- Usually inert, though some specimens fluoresce; trace elements can produce strong colour.
GEOLOGICAL SETTING
Formation
Commonly forms in metamorphosed limestone and in some igneous rocks.
MINERAL COMPANY
Common associations
Calcite, dolomite, corundum, forsterite, phlogopite, and skarn or metamorphic minerals
FIELD NOTES
How to identify it
Look for octahedral form, high hardness (7.5–8), lack of strong cleavage, and isotropic optics. Red spinel is singly refractive, unlike doubly refractive ruby.
NAME & HISTORY
Where the name comes from
From Latin spinella, probably referring to a small thorn, an allusion to pointed crystals.
SCIENTIFIC SOURCE
Handbook of Mineralogy — Spinel
External reference; factual profile paraphrased and attributed. Copyright remains with the publisher.
PROVENANCE
Documented occurrences
Kuh-i-Lal district, Ishkoshim · Historic gem spinel is documented in magnesian marble with forsterite-bearing rocks. Display is deliberately limited to district-level context.
Sourced · District precisionColor Mechanism and Spectroscopic Thermal Variation of Pink Spinel Reportedly from Kuh-i-Lal, Tajikistan