Vickers hardness testing measures how strongly a material resists permanent indentation. A diamond pyramid is pressed into a polished surface under a set load, and the size of the dent it leaves gives a number on a continuous scale. Unlike the Mohs scale, which only ranks minerals in order, Vickers figures show how much harder one material is than another. Engineers and metallurgists use the test all the time. Gemologists almost never do.
Scratch hardness and indentation hardness
Hardness has more than one meaning. Mohs measures scratch hardness: can mineral A leave a groove on mineral B? Indentation hardness asks a different question: how much force does it take to push a hard point into the surface and leave a permanent dent of a given size?
The two kinds of hardness track each other closely for most minerals, so the order of the Mohs reference minerals stays the same on a Vickers scale. What changes is the spacing. Indentation tests turn the uneven Mohs steps into measured gaps, and those gaps are far from equal.
Other indentation tests exist. Brinell testing uses a hard ball, and Rockwell testing measures indentation depth. Both suit metals and larger parts. Vickers and its close relative Knoop work on small samples and on very hard, brittle materials, which is why mineralogists turned to them.
How the Vickers test works
The test was developed in the early 1920s at the British engineering company Vickers. The indenter is a diamond ground into a square-based pyramid, with an angle of 136 degrees between opposite faces. The tester presses it into a flat, polished surface with a fixed force, holds it there for a set time (often 10 to 15 seconds), and lifts it away.
What remains is a small square dent. The operator measures the two diagonals of that square under a microscope and averages them. The hardness number comes from the load divided by the surface area of the dent: a small dent under a heavy load means a hard material.
The formula used is HV = 1.8544 x F / d squared, where F is the load in kilograms-force and d is the average diagonal in millimeters. The constant comes from the geometry of the 136 degree pyramid.
Microhardness testers apply loads of a few grams up to about a kilogram and are used on tiny areas, such as a single grain in a polished mineral section. Macro testers use much heavier loads on metal parts.
HV units
Results are written as a number followed by HV, often with the load added: 1500 HV0.5 means a Vickers hardness of 1500 measured with a 0.5 kgf load. The number itself is in kilograms-force per square millimeter, though by convention people write only HV. To convert to the SI unit of gigapascals, multiply by about 0.0098, so 1000 HV is close to 9.8 GPa.
The load matters. Very light loads on brittle materials often give higher readings than heavy loads (an effect called the indentation size effect), and cracks can form at the corners of the dent in glassy or brittle samples. A Vickers number without the load and the crystal orientation is only a rough guide.
The Knoop test
The Knoop test, introduced at the US National Bureau of Standards in 1939, uses a diamond pyramid stretched into a long, narrow rhombus. Its dent is about seven times longer than it is wide, and the operator measures only the long diagonal.
Knoop dents are shallow, so the test suits thin coatings and brittle materials that would crack under a Vickers point. Because the elongated dent lines up with a particular direction, Knoop testing is also good at measuring directional hardness. Kyanite, which is soft along its crystal length and much harder across it, and diamond, whose hardness varies slightly with crystal direction, have both been studied this way. Knoop numbers are written HK and are close to Vickers numbers for many materials, though not identical.
Comparison with the Mohs scale
The figures below are approximate. Published values vary with load, orientation, sample quality and the lab, so treat them as ballpark numbers, not specifications.
| Mineral | Mohs | Approximate Vickers (HV) |
|---|---|---|
| Talc | 1 | Too soft to measure well |
| Gypsum | 2 | 40-60 |
| Calcite | 3 | 100-150 |
| Fluorite | 4 | 170-200 |
| Apatite | 5 | 450-550 |
| Orthoclase | 6 | 700-800 |
| Quartz | 7 | 1100-1300 |
| Topaz | 8 | 1400-1650 |
| Corundum | 9 | 1800-2300 |
| Diamond | 10 | About 7000-10000 or more |
Two things stand out. The jump from corundum to diamond is several thousand HV, more than everything from gypsum to corundum combined. Topaz sits only modestly above quartz, while corundum is well above topaz, even though each pair is one Mohs step apart. A rank-order scale hides these differences. The extreme hardness of diamond comes from its dense network of strong carbon bonds, explained on the diamond chemistry page.
Why gemologists rarely use it
Vickers testing is common in materials science, ceramics and metallurgy. In gemology it plays almost no part, for several reasons:
- It leaves a permanent dent, and no owner wants a mark on a cut stone.
- It needs a flat, polished test area and a microscope-equipped tester.
- Hardness figures overlap between many gems, so a hardness number alone rarely identifies a stone.
- Optical and density tests identify gems without damage and with better results.
Gem labs rely on refractive index, specific gravity, spectroscopy, microscopy and advanced spectroscopic methods, all described on the gemological instruments page. Vickers and Knoop values show up mainly in mineralogy references and research papers on synthetic and treated materials.
Where jewelers meet Vickers numbers
Jewelry metals are a different story. Alloy makers and casting houses publish Vickers hardness for gold, platinum and silver alloys, often in both annealed and work-hardened states. Fine gold measures only a few tens of HV when annealed, while karat alloys measure several times higher, which is one reason yellow gold is alloyed for rings. Platinum alloys used for prongs are chosen partly for their hardness, a topic covered on the platinum page. The terms annealing, work hardening and temper are defined in the metallurgy glossary.
For gemstones, hardness is only one part of the picture. Toughness and cleavage often matter more for wear, and the gemstone durability guide brings those factors together.
Frequently asked questions
Is Vickers hardness more accurate than Mohs?
It is more informative because it measures on a continuous scale and shows the size of the gaps between materials. It is also destructive and needs lab equipment, so Mohs remains the everyday reference for gems.
What does HV stand for?
HV stands for Hardness Vickers. The number is in kilograms-force per square millimeter, usually written with the test load, such as HV0.5 or HV10.
Can a jeweler test my gemstone's hardness with a Vickers tester?
A jeweler or lab would not do this on a cut gem because the test leaves a permanent dent. Gem identification relies on non-destructive optical tests instead.
Why do sources give different Vickers numbers for the same mineral?
Readings change with the load, the crystal direction tested, surface preparation and the purity of the sample. That is why mineral references usually quote a range.