Household Scratch References
You do not need a mineralogy kit to test hardness. The objects below are reliable references because their hardness is well-characterized. Always test on an inconspicuous area of the stone first, and wipe away the scratch powder before reading the result — what looks like a scratch is sometimes just mineral dust from the reference.
| Object | Approx. Mohs | What it tests |
|---|---|---|
| Fingernail | 2.5 | If the object scratches your nail, it is harder than 2.5 |
| Copper coin (penny) | 3.0–3.5 | If it scratches the coin, it is harder than 3 |
| Brass key | 3.5–4.0 | Useful to distinguish calcite (3) from fluorite (4) |
| Steel kitchen knife blade | 5.5 | If the stone scratches steel, it is above 5.5 |
| Glass plate or window glass | 5.5 | If the stone scratches glass cleanly, it is above 5.5 |
| Steel file (hardware store) | 6.5 | If the stone scratches a file, it is above 6.5 |
| Quartz point or amethyst | 7.0 | The most useful single reference — see below |
| Corundum reference plate | 9.0 | Lab kit item; scratched only by diamond or moissanite |
How to Perform the Test
- Clean both surfaces. Grit or dust can mimic a scratch. Wipe the stone and the reference object with a dry cloth.
- Press and drag. Press the reference object firmly against the stone and drag across a small, inconspicuous area (the base or girdle edge, never the table or crown).
- Wipe and examine. Wipe away any powder. Look for a groove, not just a mark. A colored mark that wipes away is mineral dust from the reference, not a scratch.
- Reverse the test. Try scratching the reference object with the stone. A stone that scratches glass but cannot be scratched by a knife blade is between Mohs 5.5 and 6.5.
- Step inward. If results are ambiguous, move to the next reference in the table above. Narrow the band progressively.
- Combine with color and luster. Hardness alone rarely identifies a stone. Cross-reference the result with color, transparency, and any visible inclusions against the gemstone table below.
Before you scratch: read this
- Scratch testing is destructive. Always test on the least visible area of the stone. Soft or porous stones (pearl, opal, turquoise, lapis lazuli, malachite) can be permanently marked by any reference harder than them.
- Pearl tests at Mohs 2.5–4.5 — a fingernail can damage it. Identify pearl by its characteristic gritty feel when rubbed gently against a tooth edge.
- Opal at Mohs 5.5–6.5 can crack during testing if the stone is dehydrated. Examine opal visually (play of color, body tone) rather than scratch-testing.
- Emerald is Mohs 7.5–8 but is often heavily included and can shatter under pressure. Handle with care.
Gemstone Hardness Table
The table below lists common gemstones organized by Mohs hardness from hardest to softest. Within a hardness band, stones are distinguished by color, luster, crystal system, and common inclusions. Hardness alone rarely makes a definitive identification — but it eliminates most candidates quickly.
| Mohs | Gemstone(s) | Common colors | Notes |
|---|---|---|---|
| 10 | Diamond | Colorless, yellow, brown, blue, pink, red | Extreme brilliance; adamantine luster; scratches everything else |
| 9 | Ruby, Sapphire (corundum), Moissanite | Ruby: red. Sapphire: blue, pink, yellow, green, colorless. Moissanite: colorless | Corundum has a 6-sided basal parting; very dense (SG 4.0) |
| 8–8.5 | Alexandrite, Spinel, Topaz, Tanzanite (8) | Alexandrite: color-change green to red. Spinel: red, blue, pink. Topaz: blue, pink, imperial orange | Topaz has perfect basal cleavage — protect from impact. Tanzanite is pleochroic (blue-violet-burgundy) |
| 7.5–8 | Aquamarine, Emerald (beryl), Zircon | Aquamarine: blue-green. Emerald: green. Zircon: colorless, blue, orange | Beryl SG 2.72; emerald almost always included. Zircon SG 4.6–4.7 — heavy for its size |
| 7–7.5 | Garnet, Tourmaline, Tanzanite (low end) | Garnet: red, orange, green, yellow. Tourmaline: any color including bi-color | Garnet has no cleavage; tourmaline shows strong pleochroism. Tsavorite garnet is vivid green |
| 7 | Amethyst, Citrine, Rose Quartz, Smoky Quartz, Clear Quartz, Chalcedony, Agate | Quartz family covers virtually every color depending on variety | Quartz is the most common mineral on earth; check for conchoidal fracture and glass-like luster |
| 6.5–7 | Peridot, Jade (jadeite end), Spessartite Garnet | Peridot: yellow-green. Jadeite: green, white, lavender. Spessartite: orange-red | Peridot has characteristic oily luster and doubling of back facets under loupe |
| 6–6.5 | Moonstone, Labradorite, Amazonite, Sunstone (feldspar) | Moonstone: white, peach, grey with adularescence. Labradorite: grey with spectral flash | All feldspar has two cleavage planes at 90°; handle carefully. Moonstone is identified by its floating blue sheen |
| 5.5–6.5 | Opal | White, black, crystal; always with play of color in precious varieties | Opal is amorphous; identified by unique play of color. Can craze if dried out |
| 5–6 | Turquoise, Lapis Lazuli (5–6), Obsidian | Turquoise: blue-green with matrix. Lapis: deep blue with pyrite flecks | Turquoise is porous and dyes easily; matrix patterns help verify natural stone. Lapis has pyrite (gold flecks) |
| 3–4 | Calcite (3), Fluorite (4), Malachite (3.5–4) | Calcite: white, clear. Fluorite: purple, green, blue, colorless. Malachite: green with banding | Calcite fizzes in dilute acid. Fluorite shows strong fluorescence. Malachite has distinctive concentric green banding |
| 2.5–4.5 | Pearl | White, cream, peach, black, golden | Identified by tooth-grit test; organic material; do not scratch-test. Orient (surface luster) distinguishes quality |
| 2–2.5 | Selenite, Amber | Selenite: colorless/white. Amber: orange-yellow, sometimes red or green | Amber floats in saturated saltwater (SG ~1.1); selenite can be peeled in layers |
Narrowing Down Your Mystery Stone: Decision Path
Work through these questions in sequence. Each answer eliminates a large portion of the gemstone table.
- Does your fingernail scratch it? If yes: the stone is below Mohs 2.5. Candidates: selenite, amber, gypsum, talc. If no: continue.
- Does a copper coin scratch it? If yes: between Mohs 2.5 and 3. Candidates: pearl, calcite, coral. If no: continue.
- Does a steel knife blade scratch it? If yes: between Mohs 3 and 5.5. Candidates: fluorite (4), apatite (5), malachite (3.5–4), turquoise (5–6). If no: continue.
- Does the stone scratch glass? If no: the stone is at or below Mohs 5.5. If yes: the stone is above 5.5. Candidates now include the quartz family and above.
- Does an amethyst or clear quartz point scratch the stone? If yes: the stone is below Mohs 7. Candidates: moonstone, labradorite, opal, peridot. If no: the stone is at least Mohs 7. Candidates: quartz (if at 7), tourmaline, garnet, beryl, topaz, spinel, corundum, diamond.
- Does the stone scratch quartz? If yes: the stone is above Mohs 7. Now check color and density to distinguish tourmaline (7–7.5) from topaz (8) from corundum (9) from diamond (10). If no: the stone is quartz or a look-alike (chalcedony, agate, aventurine, tiger's eye).
When Hardness Is Not Enough
Two stones at the same Mohs rating can be completely different minerals. Hardness eliminates candidates but does not make a final identification. The next most useful checks you can do at home are specific gravity (weighing the stone in air and in water to estimate density) and visual properties (pleochroism in different lighting, fluorescence under a UV light, inclusions visible under a loupe).
For a definitive identification on a stone of value, a gemological laboratory assessment is the correct path. A refractometer reading (refractive index) and spectroscopic analysis can distinguish stones that look and feel identical.
Common stones you might be testing
Frequently Asked Questions
My stone scratches glass but does not scratch a steel file — what is it?
The stone is between Mohs 5.5 and 6.5. The most common gemstones in this range are moonstone, labradorite, and other feldspar varieties, as well as opal. Check for adularescence (moonstone's floating sheen) or play of color (opal). If neither applies, check the color and density against the table above.
Can a fake stone pass a hardness test?
Yes. Glass (Mohs 5.5) is used in some simulants and will be scratched by steel but will scratch softer materials. Synthetic stones have the same hardness as their natural counterparts — a lab-grown sapphire tests at Mohs 9 just like a natural one. Hardness does not distinguish natural from synthetic.
I got a scratch on my stone during testing — is it ruined?
A scratch on the pavilion edge or base is usually unnoticeable in a set stone. If the scratch is on the table or a visible facet, a gem cutter can sometimes re-polish the area. For soft stones like opal or turquoise, the scratch cannot be reversed without professional intervention.
What does it mean if neither surface is scratched?
The two materials are at approximately the same Mohs rating. This is useful information — if the stone and a steel knife blade show no scratches on either side, the stone is at approximately Mohs 5.5. Move to the next reference.
Can I test a pearl's hardness with a tooth?
The tooth-grit test (rubbing a pearl across a tooth edge) tests surface texture, not hardness. Real pearl feels gritty because its surface is made of microscopic aragonite platelets. Imitation pearl (glass or resin) feels smooth. This is the correct identification test for pearl — not a scratch test.