"A real diamond scratches glass." True. So does quartz, topaz, corundum, spinel, cubic zirconia, moissanite and every glass harder than the window pane. The scratch test is the most widely known and most misapplied idea in gem identification, and a laboratory almost never uses it — because it damages the stone, and because it rarely answers the question being asked.
The Mohs scale
Friedrich Mohs ranked ten minerals by which could scratch which, from talc (1) to diamond (10):
| Mohs | Reference mineral | Gems at this level |
|---|---|---|
| 10 | Diamond | Diamond |
| 9 | Corundum | Ruby, sapphire (moissanite ~9.25) |
| 8 | Topaz | Topaz, spinel, cubic zirconia (~8.5), chrysoberyl (8.5) |
| 7 | Quartz | Amethyst, citrine, agate, tourmaline (7–7.5), garnet (6.5–7.5), zircon (7.5) |
| 6 | Orthoclase | Moonstone, jadeite (6.5–7), nephrite (6–6.5), opal (5.5–6.5) |
| 5 | Apatite | Apatite, turquoise (5–6), glass (~5.5) |
| 4 | Fluorite | Fluorite, rhodochrosite |
| 3 | Calcite | Coral, pearl (2.5–4.5), malachite (3.5–4) |
| 2 | Gypsum | Amber (2–2.5), selenite |
| 1 | Talc | — |
The scale is ordinal, not proportional. The gap between 9 and 10 is far larger in absolute terms than between 1 and 2; diamond is several times harder than corundum, not one step harder.
Why the laboratory does not scratch
It is destructive. Every scratch is permanent damage to the stone or to the reference plate. Non-destructive testing is the whole basis of gem identification, and hardness can be inferred from identity rather than tested for.
It is ambiguous. A scratch on glass proves only that the stone is harder than glass — about 5.5 — which describes most gem materials and most of their imitations. Cubic zirconia scratches glass. Synthetic sapphire scratches glass. A scratch test cannot separate any of them.
It can be misread. A soft stone drawn across a hard plate leaves a mark that looks like a scratch but is powder from the stone itself. Wiping the plate shows the difference; many home testers never wipe.
It can be mis-applied to the wrong stone. Scratching a suspected imitation with a known hard point, rather than the reverse, is safer — but only if the suspected stone is worthless, and the whole point is that you do not yet know.
What hardness is useful for
Wear and care. Hardness predicts how a stone survives daily wear. Anything below 7 will be scratched by household dust (which contains quartz) over time; opal, turquoise, pearl and coral need protection. Daily-wear care for gemstone jewellery.
Recognising damage. Fine scratches on the surface of a "sapphire" that has been worn for a year mean it is not sapphire, which at 9 does not scratch in ordinary use.
Explaining a report. The hardness on a DRGTL gem report is a property of the identified species, given for the buyer's information, not the result of a test performed on the stone.
Hardness is not toughness
Hardness is resistance to scratching. Toughness is resistance to breaking. They are different properties and often opposed: diamond is the hardest material and yet cleaves along certain planes with a sharp blow; jadeite at 6.5–7 is one of the toughest gem materials known because of its interlocking fibrous structure. An emerald at 7.5–8 is brittle and chips at the corners; a nephrite carving at 6 survives generations of handling.
Buyers who ask "is it hard enough to wear?" are usually asking about both, and a report that gives hardness alone answers half the question.
Frequently asked questions
Is the scratch test ever appropriate?
On rough material of negligible value, as a rough field check, by someone who understands the limits. On any cut stone of value, no.
A stone scratched glass. Is it a diamond?
It is harder than glass. So is almost everything sold as a diamond simulant.
Can hardness be measured without scratching?
Not directly. But identity determines hardness, and identity is measured by RI, SG and the microscope. Once the species is known, its hardness is known.
Which gems are too soft to wear daily?
As a rough line, anything below 7: opal, turquoise, moonstone, pearl, coral, amber, malachite, fluorite. They can be worn with care and stored separately.
Does hardness affect value?
Indirectly, through durability. But a soft opal can be worth far more than a hard piece of quartz. Hardness describes the material, not the market.