Ruby Identification Guide

What Ruby Looks Like

Ruby is the red gem variety of corundum (aluminum oxide, Al2O3), colored by chromium. Only red corundum is ruby; other colors are sapphire.

• Color: Red, from pinkish-red to deep "pigeon's blood" crimson.
• Luster: Vitreous to subadamantine.
• Transparency: Transparent to translucent.
• Habit: Hexagonal, often tabular or barrel-shaped (steep bipyramidal) crystals; sometimes flat plates.
• Special optic: Some rubies show asterism — a six-rayed star — from rutile silk when cut as a cabochon.

Step-by-Step Field-ID Checklist

1. Assess hardness. Ruby is Mohs 9 — it scratches almost everything and is not scratched by quartz, garnet, or a steel file.
2. Check crystal form. Hexagonal tabular or barrel shapes with flat ends suggest corundum.
3. Feel the weight. High density (~4.0) makes ruby feel heavy for its size.
4. Use a UV light. Many natural (especially chromium-rich Burmese) rubies fluoresce red under UV.
5. Inspect inclusions — natural ruby often shows fine rutile "silk," crystals, and growth zoning; flawless cheap "rubies" are usually glass or synthetic.

Key Diagnostic Tests

• Mohs hardness: 9 — second only to diamond among common gems.
• Streak: White (the mineral is harder than the streak plate).
• Cleavage: None; parting and conchoidal fracture.
• Density: ~3.97-4.05 g/cm3 — distinctly heavy.
• Fluorescence: Often strong red under UV.
• Acid: Inert.

Common Look-Alikes and How to Tell Them Apart

• Red garnet (almandine/pyrope): Softer (6.5-7.5), isotropic (no pleochroism), no red fluorescence; ruby is harder and doubly refractive with pleochroism.
• Rubellite (red tourmaline): Much softer (7-7.5), striated triangular prisms, lower density.
• Red spinel: Slightly softer (8), isotropic, octahedral crystals; historically confused with ruby ("Black Prince's Ruby").
• Glass imitations: Soft (~5-6), warm to touch, round bubbles, mold seams.
• Synthetic ruby: Same properties as natural; tell by curved growth striae and gas bubbles instead of natural rutile silk.

Where It Is Found

Ruby forms in marbles and basalt-related deposits. Premier sources include Myanmar (Mogok), Mozambique, Thailand, Sri Lanka, Madagascar, and Afghanistan.

Field Tips and Common Mistakes

Hardness and density are the workhorses of ruby identification. Only diamond, moissanite, and another corundum will scratch a true ruby, so a stone that is easily marked by a quartz point or a steel file is not ruby. Combine this with the heft test — ruby's density near 4.0 makes it feel heavier than a same-size garnet, tourmaline, or glass. A long-wave UV light adds confidence, since many chromium-rich rubies glow red.

The most common mistakes are confusing ruby with red garnet, spinel, or glass, and overlooking synthetics. Garnet and spinel are isotropic and lack ruby's pleochroism; glass is soft and bubbly. Synthetic ruby shares ruby's hardness and chemistry, so look instead at inclusions: natural ruby shows fine rutile silk, crystals, and angular growth zoning, while flame-fusion synthetics show curved growth striae and round gas bubbles. When value is at stake, a lab report is the only certainty.