Diopside
Diopside is a calcium magnesium silicate pyroxene mineral, CaMgSi2O6, common in metamorphosed limestones and mafic to ultramafic igneous rocks. It typically forms stubby prismatic crystals with a nearly square cross-section, a hallmark of the pyroxene group's characteristic cleavage angles. Diopside's color ranges from colorless to deep green depending on iron and chromium content, and chromium-rich varieties are prized as gem material.
Diopside properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | CaMgSi2O6 |
| Mohs hardness | 5.5–6.5 |
| Crystal system | Monoclinic |
| Luster | Vitreous |
| Streak | White |
| Cleavage | Good prismatic (~87°/93°) |
| Fracture | Conchoidal to uneven |
| Specific gravity | 3.2–3.3 |
| Transparency | Transparent to translucent |
| Common colors | Light to dark green, White, Gray, Blue-violet (violane), Black (chromian varieties) |
| Rarity | common |
Geology & background
Diopside forms in two principal geologic settings: as a product of contact or regional metamorphism of impure dolomitic limestones, where it crystallizes alongside forsterite, calcite, and phlogopite mica, and as a primary mineral in basalts, gabbros, and peridotites where it crystallizes directly from mafic magma. Notable localities include the marbles of DeKalb, New York, which produce fine diopside crystals; the chrome diopside deposits of Siberia's Inagli and Sakha regions, prized for their vivid green gem material; and the kimberlite pipes of South Africa, where diopside occurs as a mantle-derived xenocryst. Diopside is also a common constituent of eclogite and other high-pressure metamorphic rocks.
Rockhounds should note that gem-quality chrome diopside from Russia has become widely available and affordable compared to most green gemstones, making it a popular entry point for colored stone collecting, though its relatively low hardness limits its use in rings and other high-wear jewelry. Well-formed prismatic crystals from skarn and marble localities, showing the pyroxene group's characteristic stubby habit, are valued specimens for mineral collections illustrating contact metamorphism.
How to identify Diopside
Diopside ranges from colorless and pale green to deep chrome-green or brown, with a vitreous luster and a white to pale green streak. It has a hardness of 5.5 to 6.5 and two cleavage directions meeting at nearly ninety degrees, a signature trait of the pyroxene group that distinguishes it from amphiboles, whose cleavage meets at about 56 and 124 degrees. Diopside is commonly confused with augite and hornblende, but its lighter color, pyroxene-angle cleavage, and typical occurrence in marble or basalt help separate it from these related minerals.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Diopside is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Diopside worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Chrome diopside from Siberia owes its vivid emerald-green color to trace chromium and is sometimes marketed as "Russian diopside" in the gem trade.
- Diopside belongs to the pyroxene group, whose name comes from Greek words meaning "fire" and "stranger," reflecting early mineralogists' surprise at finding these crystals in volcanic rock.
- Star diopside, a rare cat's-eye or four-rayed variety, gets its optical effect from oriented needle-like inclusions of magnetite or ilmenite.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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