Chloro-spinel is the recognised varietal name for green spinel, a member of the spinel group with the base composition MgAl₂O₄. The prefix chloro- derives from the Greek khlōros, meaning green or yellow-green, and distinguishes this colour variety from the more commercially prominent red (rubicelle and flame spinel), blue, and lavender spinels. Colour in chloro-spinel arises principally from iron, with chromium contributing in finer, more saturated specimens; the interplay of these two chromophores determines whether a stone reads as a clean, vivid green or carries the greyish or brownish secondary hues that characterise the majority of material in the market. Genuinely fine chloro-spinel — strongly saturated, minimally modified green — is among the rarer expressions of the spinel species.

Mineralogy and Chemistry

Spinel belongs to the cubic crystal system and the spinel supergroup, crystallising most commonly as octahedra, sometimes twinned into the characteristic spinel law contact twins. The ideal end-member formula is MgAl₂O₄, but natural spinels accommodate a wide range of substitutions across the solid-solution series. In chloro-spinel, iron (Fe²⁺ and Fe³⁺) substitutes for magnesium and aluminium within the structure; chromium (Cr³⁺) may additionally substitute for aluminium. Iron alone tends to produce muted, yellowish or greyish greens; the presence of chromium shifts the hue toward a purer, more saturated green, analogous to chromium's role in emerald and demantoid garnet.

Key physical and optical constants are consistent with the broader spinel species:

• Crystal system: Cubic (isometric)
• Hardness (Mohs): 8
• Specific gravity: approximately 3.58–3.61
• Refractive index: approximately 1.712–1.736 (singly refractive, as expected for a cubic mineral)
• Lustre: vitreous
• Cleavage: none; conchoidal fracture

Because spinel is singly refractive, it lacks the birefringence that can cause optical blurring in doubly refractive stones, and this property — combined with its hardness and lack of cleavage — makes it an excellent candidate for faceting.

Colour Range and Appearance

The colour palette of chloro-spinel extends from pale mint and celadon greens through mid-toned olive and forest greens to, occasionally, a deep, richly saturated green that invites comparison with fine tsavorite garnet or chrome tourmaline. In practice, the majority of green spinel on the market sits in the moderate-saturation range with a noticeable grey or brownish modifier, a direct consequence of iron's tendency to absorb across multiple wavelength bands. Stones with a dominant chromium contribution are distinctly rarer and command a meaningful premium.

Under long-wave and short-wave ultraviolet illumination, iron-dominant green spinels are typically inert, whereas chromium-bearing specimens may display a weak to moderate red fluorescence — a useful diagnostic indicator of chromium participation in the colouring mechanism.

Principal Localities

Green spinel occurs in the same geological settings as other gem spinels: marble-hosted and skarn deposits, as well as alluvial gravels derived from such primary sources.

• Myanmar (Burma): The Mogok Stone Tract, historically the world's pre-eminent spinel locality, produces green spinel alongside its celebrated red and pink material, though green stones represent a small fraction of total output.
• Tanzania: The Mahenge and Tunduru regions yield green spinel, sometimes of good saturation. Tanzanian material is increasingly well-regarded in the trade.
• Vietnam: Alluvial deposits in Lục Yên and Quỳ Châu have produced green spinels, occasionally with attractive chromium-influenced colour.
• Sri Lanka: Elahera and other localities yield green spinel, though much Sri Lankan material tends toward paler, less saturated tones.
• Tajikistan: The Pamir region, historically known for red spinel (Balas ruby), also produces occasional green material.

Treatment

Spinel as a species is notable for being one of the few gem materials routinely encountered in an untreated state. Unlike corundum, which is almost universally heat-treated, the great majority of spinel — including green spinel — reaches the market without thermal or chemical enhancement. Heat treatment of spinel is not a standard commercial practice, and no established filler or fracture-healing treatments are documented for the species in the way that lead-glass filling is for ruby or oiling is for emerald. This relative freedom from treatment is a genuine commercial asset and is increasingly acknowledged by laboratories and auction houses in their grading reports.

Gemmological laboratories including GIA and Gübelin issue reports for spinel that address treatment status; for fine chloro-spinel, a laboratory report confirming no indications of treatment adds measurable value in the current market.

Distinction from Similar Stones

Chloro-spinel may be confused with several other green gem materials, particularly in less saturated or more yellowish tones:

• Tsavorite garnet: Singly refractive like spinel but with a higher refractive index (approximately 1.739–1.744) and higher specific gravity (approximately 3.61–3.68). Tsavorite typically displays stronger, cleaner green saturation and a characteristic absorption spectrum.
• Chrome tourmaline: Doubly refractive, with strong pleochroism; easily separated from spinel by polariscope and refractometer.
• Peridot: Doubly refractive with a distinctive yellowish-green body colour and characteristic absorption bands; specific gravity approximately 3.34, lower than spinel.
• Green sapphire: Doubly refractive (uniaxial negative); hardness 9; distinct absorption spectrum.
• Demantoid garnet: Higher dispersion, lower refractive index than spinel; characteristic horsetail inclusions in Uralian material.

The combination of cubic singly refractive optics, a refractive index near 1.72, hardness of 8, and specific gravity near 3.60 is diagnostic for spinel as a species; colour then places a green specimen within the chloro-spinel variety.

Market Context

Within the spinel market, red and vivid pink-to-orange spinels (including the celebrated hot pink and flame colours) command the highest prices, followed by fine cobalt-blue and violet material. Green spinel occupies a more modest position, reflecting both its relative abundance in lower-saturation grades and the comparative rarity of truly fine, vivid examples. A well-saturated, chromium-influenced green spinel of clean clarity and good cut can nonetheless achieve prices that reflect its genuine scarcity, and collector interest in unusual spinel colours has grown appreciably since spinel's removal from the corundum composite listing in major gem guides and its increasing recognition as a distinct, desirable species in its own right.

Chloro-spinel is not yet a widely marketed commercial category in the way that Mahenge spinel or cobalt spinel are, but it is correctly identified as such in GIA educational materials and gemmological literature. Dealers specialising in collector-grade spinels increasingly present fine green material under this varietal name.

Further Reading

• GIA Gem Encyclopedia: Spinel
• Gems & Gemology (GIA research journal)
• Lotus Gemology: Spinel Reference