Spinel Identification Guide

Identify spinel by its octahedral crystals, single refraction, hardness 8, color range, and how to separate it from ruby and sapphire.

Read the full Spinel encyclopedia entry →

What Spinel Looks Like

Spinel (magnesium aluminum oxide, MgAl₂O₄) is a hard, brilliant gem that occurs in nearly every color — most famously a vivid red that historically was confused with ruby ('Balas ruby'), plus pink, orange, purple, blue, violet, and black. It has a vitreous luster and excellent transparency in gem material.

Color: red, pink, orange, purple, blue, violet, gray, black
Transparency: transparent to opaque
Habit: isometric — sharp octahedra are the classic and diagnostic crystal form; also rounded pebbles in gem gravels

Step-by-Step Field-ID Checklist

Look for octahedral crystals (eight triangular faces, like two pyramids base-to-base) — a strong spinel signature.
Assess color and clarity — spinel is often very clean, with bright, slightly soft (non-neon) hues.
Test single vs double refraction. Spinel is singly refractive (isometric): through a loupe, back facet edges appear single, not doubled. This separates it from corundum (ruby/sapphire), which is doubly refractive.
Check for inclusions — spinel commonly shows octahedral negative crystals or strings of tiny octahedra.
Confirm hardness by scratching glass and topaz behavior as below.

Key Diagnostic Tests

Mohs hardness: 8 — scratches quartz and topaz-test material; very durable.
Streak: white.
Cleavage: none (parting may occur); fracture conchoidal.
Density: ~3.58–3.61 g/cm³ for common spinel.
Refractive index: ~1.712–1.736, singly refractive (no birefringence/doubling).
Fluorescence: many red/pink spinels glow red under UV.
Magnetism: some spinels (especially iron/zinc-bearing and 'cobalt' blue) show a weak response to a strong neodymium magnet — a useful modern gem test.
No acid reaction.

Common Look-Alikes and How to Tell Them Apart

Ruby (corundum): the classic confusion. Ruby is doubly refractive (facet doubling visible) and harder (9), forms hexagonal/tabular crystals, and is often more strongly dichroic; red spinel is singly refractive with octahedral habit and shows no pleochroism.
Garnet (pyrope/almandine): also singly refractive and red, but garnet is softer (6.5–7.5) and denser (~3.8–4.2); spinel's hardness of 8 and octahedral crystals separate it.
Sapphire (blue corundum): doubly refractive and harder than blue spinel.
Synthetic spinel: common; look for curved growth striae and gas bubbles, and an often 'too clean' appearance — lab work may be needed.
Glass imitations: singly refractive too, but softer (5–6), with gas bubbles and swirl marks, and warmer to the touch.

Where It Is Found

Gem spinel comes from Myanmar (Mogok), Tanzania (Mahenge, famous for hot-pink/red), Sri Lanka, Vietnam (Luc Yen), Tajikistan (Kuh-i-Lal), and Madagascar. It forms in marbles (contact metamorphism of impure limestone) and is recovered from alluvial gem gravels.

Frequently asked questions

How can you tell if it's real spinel?

Real spinel is singly refractive (no facet doubling), hardness 8, has no cleavage with conchoidal fracture, often forms octahedral crystals, and has a density near 3.6. Many red/pink stones fluoresce red under UV, and some show a weak pull to a strong magnet.

Spinel vs ruby — how do I tell them apart?

Ruby (corundum) is doubly refractive, so back facets appear doubled through a loupe, it is harder (9), and it is pleochroic; red spinel is singly refractive, shows no pleochroism, and often grows as octahedra. Historically many 'rubies' were actually spinel.

What colors does spinel come in?

Almost every color: red, vivid pink, orange, purple, violet, blue, gray, and black.

Is spinel magnetic?

Pure magnesium-aluminum spinel is essentially non-magnetic, but iron-, zinc-, and cobalt-bearing spinels can show a weak attraction to a strong neodymium magnet, which gemologists use as a quick screening test.

Spinel vs garnet — how do I tell them apart?

Both are singly refractive and can be red, but spinel is harder (8 vs garnet's 6.5–7.5) and less dense (~3.6 vs ~3.8–4.2), and spinel forms octahedra while garnets form dodecahedra.

Spinel identified by the community

Recent Spinel specimens identified with Rock Identifier.