Ruby
Al₂O₃
COMPARE
Side by side
Put up to four minerals or gems next to each other and test them like a gemmologist.
Spinel
MgAl₂O₄
Garnet
X₃Y₂(SiO₄)₃
VERDICT
How to tell them apart
THE TESTS
Gem-testing instruments
Scratch test
Mohs hardness: a mineral scratches anything softer than itself.
Heavy liquid
Density in g/cm³: denser stones sink deeper.
- Ruby 3.98–4.1
- Spinel 3.6–4.1
- Garnet 3.5–4.3
Refractometer
Refractive index: how strongly the stone bends light. See light bend through a 3D crystal →
1.401.451.501.551.601.651.701.751.80
- Ruby 1.759–1.772
- Spinel 1.719
- Garnet 1.73–1.89
Polariscope
Between crossed polarisers a doubly refractive stone blinks as it turns; a singly refractive one stays dark. See light bend through a 3D crystal →
Crystal structures
Unit cells side by side. Drag any of them: they all turn together.
RubyTrigonal
SpinelCubic
GarnetCubic
PROPERTIES
All properties
| Ruby | Spinel | Garnet | |
|---|---|---|---|
| Type | Gem | Gem | Gem |
| Mineral | Corundum | Spinel | Garnet group |
| Formula | Al₂O₃ | MgAl₂O₄ | X₃Y₂(SiO₄)₃ |
| Mineral group | Oxides | Oxides | Silicates — garnet group |
| Crystal system | Trigonal | Cubic | Cubic |
| Mohs hardness | 9 | 7.5–8 | 6.5–7.5 |
| Density | 3.98–4.10 g/cm³ | 3.6–4.1 g/cm³; composition controls the range | Common gem garnets roughly 3.5–4.3 g/cm³ depending on species |
| Refractive index | nω about 1.767–1.772; nε about 1.759–1.763 | n ≈ 1.719 for Mg-rich spinel; rises with Fe, Zn, and Cr substitution | n commonly about 1.73–1.89 across the group |
| Optical class | Uniaxial (−) | Isotropic | Isotropic |
| Colour | Red to purplish red | Red and pink are classic; also blue, violet and other colours | Red is classic; also orange, green, yellow and purple |
| Colour cause | Chromium is the trace element behind ruby's red, which ranges from orangy red to purplish red; more chromium gives a stronger red. Chromium also makes ruby fluoresce red, which intensifies its colour. Rubies formed in marble, such as those from Myanmar, the Himalayas and northern Vietnam, contain little iron, so their red is especially intense. | Red spinel is coloured by chromium, the same trace element that colours ruby and emerald. | — |
| Treatments | Ruby can be treated to change its colour, apparent clarity or durability. GIA research has documented low-temperature heating, flux healing of fissures, and filling of fractures with glass. | — | — |
| Look-alikes | Red spinel is coloured by the same element, chromium, and was mistaken for ruby for centuries (see Spinel). Laboratory-grown ruby, with essentially the same chemical, physical and optical properties, also exists. | Spinel was mistaken for ruby for centuries, and many famous historical "rubies" are spinel — among them the Black Prince's Ruby set in the front of Britain's Imperial State Crown and the Timur Ruby, which weighs over 350 carats and carries several Persian inscriptions. The two were separated scientifically only in 1783, when Jean-Baptiste Louis Romé de l'Isle distinguished them by their crystal shapes (GIA reading list). | — |
| Crystal habit | Barrel-shaped, pyramidal, tabular, or prismatic crystals; also granular and massive. Parting and growth features may be prominent. | Octahedral crystals are classic; also dodecahedral, granular, or massive. Contact twins on {111} define the spinel law. | Typically dodecahedral or trapezohedral crystals; also granular or massive. This group profile uses almandine as a representative reference. |
| Cleavage | No true cleavage; common basal and rhombohedral parting | Imperfect parting on {111}; true cleavage is indistinct | None |
| Fracture | Uneven to conchoidal | Conchoidal to uneven or splintery | Conchoidal to uneven |
| Tenacity | Brittle, but compact material is tough | Brittle | Brittle |
| Luster | Adamantine to vitreous | Vitreous | Vitreous to resinous |
| Streak | White | White | White |
| Transparency | Transparent to opaque | Transparent to opaque | Transparent to opaque |
| Fluorescence | Variable; ruby may fluoresce red under ultraviolet light and some material phosphoresces. | Usually inert, though some specimens fluoresce; trace elements can produce strong colour. | Usually inert to variable; fluorescence is not a group-level diagnostic. |
| Identification | Its hardness of 9 is decisive: it scratches nearly every common mineral. High density and parting help distinguish it; ruby and sapphire are colour varieties of corundum, not separate mineral species. | 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. | Equant dodecahedral habit, absent cleavage, relatively high hardness and density are useful clues; refractive index and chemistry are needed to identify the garnet species. |
| In Tajikistan | Gem ruby is documented from the marble-hosted Snezhnoe deposit in the Kukurt district, Eastern Pamir. | Kuh-i-Lal in the Ishkoshim district is the historic source of gem spinel, the "balas ruby" of Badakhshan. | Well-formed almandine crystals, up to about 7 cm, occur in garnet-bearing schist horizons of the Sokhcharv valley, Western Pamir. |
| Source | Handbook of Mineralogy — Corundum | Handbook of Mineralogy — Spinel | Handbook of Mineralogy — Almandine (representative garnet species) |