Hardness picks — also known as Mohs picks — are a set of metal-tipped probes, each pointed with a material of precisely known hardness on the Mohs scale, used to determine the relative hardness of an unknown mineral or gemstone by scratch testing. A standard set typically spans Mohs values 5 through 9, corresponding to reference minerals such as apatite (5), orthoclase feldspar (6), quartz (7), topaz (8), and corundum (9). By systematically attempting to scratch the specimen's surface with each pick in ascending order, the gemmologist can bracket the material's hardness and thereby narrow its identity considerably.

Principle of use

The Mohs hardness scale, devised by the German mineralogist Friedrich Mohs in 1812, ranks minerals on a relative, non-linear scale of scratch resistance from 1 (talc) to 10 (diamond). A material of higher hardness will scratch one of lower hardness; a material of equal hardness may or may not leave a mark depending on pressure and angle. Hardness picks exploit this principle in a portable, self-contained format. To test a specimen, the gemmologist selects a pick of known hardness and draws its tip firmly across an inconspicuous surface — ideally a natural face, a girdle edge, or a broken surface rather than a polished facet. If the pick leaves a groove, the specimen is softer than the pick's rated hardness; if it does not, the specimen is equal to or harder than the pick. Working through the set in sequence allows the tester to establish a hardness range, for example confirming that a stone is scratched by the Mohs-8 topaz pick but not by the Mohs-7 quartz pick, placing it between 7 and 8 — consistent with, among other possibilities, beryl (7.5–8).

Construction and materials

Commercial hardness pick sets are typically housed in pen-like aluminium or steel handles, colour-coded or numbered for each hardness value. The tips themselves are bonded fragments or sintered compacts of the reference mineral, or in some cases synthetic equivalents of equivalent hardness — synthetic corundum, for instance, serves reliably as the Mohs-9 tip. The tips are replaceable in quality sets, since repeated use gradually dulls the point and reduces testing reliability. Some professional sets include a Mohs-10 diamond-tipped pick, extending the range to the full upper end of the scale, though this is less commonly needed in routine gem identification given that diamond is readily identified by other means.

Applications and limitations

Hardness picks are valued primarily for field identification — at mineral shows, in the field during prospecting, or in situations where laboratory instruments are unavailable. They are compact, require no power source, and provide immediate, tactile results. In a gemmological laboratory context, they serve as a rapid preliminary filter before more definitive optical or spectroscopic testing.

Their limitations are significant, however, and experienced gemmologists apply them with considerable caution:

• Surface damage. Scratch testing is inherently destructive. Even a brief test on a polished facet can leave a visible abrasion, reducing the value of a finished gem. Testing should always be confined to inconspicuous areas.
• Directional hardness. Many minerals exhibit hardness anisotropy — kyanite being the classic example, with a Mohs hardness of approximately 4.5 along its length but 6.5 across it. A single scratch test on one face may therefore be misleading.
• Confounding powders. A scratch mark may in fact be a streak of powder from the pick tip rather than a true groove in the specimen. The test area should be wiped clean and examined with a loupe before drawing conclusions.
• Composite and assembled stones. Doublets and triplets may yield anomalous results if the pick contacts a cement layer or a different component than intended.
• Coatings and treatments. Surface coatings, lacquers, or wax treatments can produce misleadingly low hardness readings.

For these reasons, hardness picks are considered a supporting tool rather than a definitive identifier, and their results are always interpreted alongside other observations — refractive index, specific gravity, optical character, and spectroscopic data where available.

In the trade

Hardness picks remain a standard component of a field gemmologist's kit alongside a hand loupe, dichroscope, and Chelsea filter. They are particularly useful when assessing rough material of uncertain identity, where surface damage is less consequential than with cut and polished stones. In the context of estate jewellery assessment or gem fair purchasing, a quick hardness check can help distinguish, for example, a glass imitation (Mohs ~5.5) from a genuine quartz or feldspar simulant, or confirm that a suspected ruby is not red glass before committing to further testing.