Diamond Identification Guide

What Diamond Looks Like

Diamond is pure crystalline carbon, the hardest natural material. Rough natural diamond often forms octahedral (eight-sided) crystals, sometimes cubes or dodecahedra, with characteristic rounded, stepped faces and trigons (triangular etch pits). It ranges from colorless to yellow, brown, gray, and rarely pink, blue, or green, with an unmistakable adamantine (brilliant, almost metallic-bright) luster and a greasy feel on rough faces.

Key visual cues:

• Luster: adamantine — exceptionally bright, high reflectivity.
• Crystal habit: octahedra, cubes, dodecahedra; curved faces in rough.
• Transparency: transparent to translucent.
• Surface: trigons and a slightly greasy luster on natural rough.

Step-by-Step Field-ID Checklist

1. Look for crystal form — natural octahedra with trigons strongly suggest diamond.
2. Note the luster — adamantine brilliance unlike any common mineral.
3. Hardness test (with care). Diamond (Mohs 10) scratches everything, including corundum; nothing common scratches it.
4. Check for greasy feel on rough surfaces and water-shedding (diamond is hydrophobic — water beads up).
5. Use a diamond tester (thermal/electrical conductivity) for cut stones.

Diagnostic Tests

• Mohs hardness: 10 — defining and unmatched; it scratches sapphire/corundum.
• Streak: white/colorless (must be tested on hard plate; it won't mark a normal streak plate).
• Cleavage: perfect octahedral cleavage in four directions.
• Density: ~3.52 g/cm³.
• Thermal conductivity: extremely high — diamond testers exploit this (note: moissanite also conducts; use a combined tester).
• Luster/dispersion: adamantine with high 'fire' (color flashes).
• Hydrophobic: water beads and a fog test clears almost instantly.

Common Look-Alikes and How to Tell Them Apart

• Cubic zirconia (CZ): denser (~5.7) so heavier for its size, softer (Mohs 8), and fails a thermal tester; lacks diamond's crisp adamantine luster.
• Moissanite: nearly as hard, but doubly refractive (look for doubled facet edges under a loupe) and shows more rainbow fire; passes thermal but fails a dedicated moissanite tester.
• White sapphire/quartz/topaz: all softer (Mohs 7–9); diamond will scratch them, and they lack diamond's fire and brilliance.
• Glass: much softer (~5.5), warm to touch, with rounded mold facets and bubbles.
• Lab-grown diamond: chemically identical real diamond — distinguished only by gemological lab testing, not field tests.

Where It Is Found

Natural diamond crystallizes deep in the mantle and is carried up in kimberlite and lamproite pipes, then concentrated in alluvial/placer gravels. Major sources include South Africa, Botswana, Russia, Canada, Australia (Argyle), and Brazil; in the U.S., Crater of Diamonds State Park (Arkansas) lets visitors search a diamond-bearing pipe. Field diamonds are most often found as waterworn crystals in stream and beach gravels.