Peridot
Mg₂SiO₄
COMPARE
Side by side
Put up to four minerals or gems next to each other and test them like a gemmologist.
Emerald
Be₃Al₂Si₆O₁₈
Garnet
X₃Y₂(SiO₄)₃
Spinel
MgAl₂O₄
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.
- Peridot 3.27
- Emerald 2.63–2.97
- Garnet 3.5–4.3
- Spinel 3.6–4.1
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
- Peridot 1.635–1.67
- Emerald 1.564–1.602
- Garnet 1.73–1.89
- Spinel 1.719
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.
PeridotOrthorhombic
EmeraldHexagonal
GarnetCubic
SpinelCubic
PROPERTIES
All properties
| Peridot | Emerald | Garnet | Spinel | |
|---|---|---|---|---|
| Type | Gem | Gem | Gem | Gem |
| Mineral | Forsterite | Beryl | Garnet group | Spinel |
| Formula | Mg₂SiO₄ | Be₃Al₂Si₆O₁₈ | X₃Y₂(SiO₄)₃ | MgAl₂O₄ |
| Mineral group | Silicates — olivine group | Silicates | Silicates — garnet group | Oxides |
| Crystal system | Orthorhombic | Hexagonal | Cubic | Cubic |
| Mohs hardness | 7 | 7.5 | 6.5–7.5 | 7.5–8 |
| Density | 3.275 g/cm³ | 2.63–2.97 g/cm³ | Common gem garnets roughly 3.5–4.3 g/cm³ depending on species | 3.6–4.1 g/cm³; composition controls the range |
| Refractive index | nα 1.635; nβ 1.651; nγ 1.670 | nω about 1.568–1.602; nε about 1.564–1.595 | n commonly about 1.73–1.89 across the group | n ≈ 1.719 for Mg-rich spinel; rises with Fe, Zn, and Cr substitution |
| Optical class | Biaxial (+) | Uniaxial (−) | Isotropic | Isotropic |
| Colour | Yellowish green; the finest is a pure grass green | Green to bluish green | Red is classic; also orange, green, yellow and purple | Red and pink are classic; also blue, violet and other colours |
| Colour cause | Unlike most gems, which owe their colour to impurities, peridot is green because of its own composition; olivines closer to the forsterite end tend to be greener. | — | — | Red spinel is coloured by chromium, the same trace element that colours ruby and emerald. |
| Treatments | — | Emeralds typically contain inclusions visible to the unaided eye, often called jardin ("garden"). GIA's laboratory classifies emerald clarity treatments as minor, moderate or significant. | — | — |
| Look-alikes | Green glass is the most common peridot imitation. Synthetic corundum and spinel and cubic zirconia may be passed off as peridot, and natural emerald and demantoid garnet can be confused with it. | Green beryl whose colour is too light is not called emerald, although it is the same mineral. | — | 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 | Euhedral to subhedral thick crystals with wedge-shaped terminations; commonly granular or compact massive. | Usually hexagonal prisms, sometimes tabular; crystals may be vertically striated, etched, or exceptionally large in pegmatites. | Typically dodecahedral or trapezohedral crystals; also granular or massive. This group profile uses almandine as a representative reference. | Octahedral crystals are classic; also dodecahedral, granular, or massive. Contact twins on {111} define the spinel law. |
| Cleavage | Imperfect on {010} and {100} | Imperfect basal cleavage | None | Imperfect parting on {111}; true cleavage is indistinct |
| Fracture | Conchoidal | Conchoidal to uneven | Conchoidal to uneven | Conchoidal to uneven or splintery |
| Tenacity | Brittle | Brittle | Brittle | Brittle |
| Luster | Vitreous | Vitreous to resinous | Vitreous to resinous | Vitreous |
| Streak | White | White | White | White |
| Transparency | Transparent to translucent | Transparent to translucent | Transparent to opaque | Transparent to opaque |
| Fluorescence | No diagnostic fluorescence is reported in the cited profile. | Usually weak or inert; response varies with colour centres and impurities. | Usually inert to variable; fluorescence is not a group-level diagnostic. | Usually inert, though some specimens fluoresce; trace elements can produce strong colour. |
| Identification | Hardness 7, conchoidal fracture, granular habit and olive-green colour are useful field clues. Optical properties and chemistry distinguish magnesium-rich forsterite from iron-rich olivine members. | Recognise hexagonal prisms, hardness 7.5–8, relatively low density, and imperfect basal cleavage. Emerald and aquamarine are coloured beryl, sharing the same ring-silicate framework. | Equant dodecahedral habit, absent cleavage, relatively high hardness and density are useful clues; refractive index and chemistry are needed to identify the garnet 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. |
| In Tajikistan | — | — | Well-formed almandine crystals, up to about 7 cm, occur in garnet-bearing schist horizons of the Sokhcharv valley, Western Pamir. | Kuh-i-Lal in the Ishkoshim district is the historic source of gem spinel, the "balas ruby" of Badakhshan. |
| Source | Handbook of Mineralogy — Forsterite | Handbook of Mineralogy — Beryl | Handbook of Mineralogy — Almandine (representative garnet species) | Handbook of Mineralogy — Spinel |