Guides / hardness

Mohs Hardness Test Kit: What to Buy or Build at Home

October 19, 2026 / 7 min read / By Viral Tandel

You have a pile of rocks on the kitchen table and a sudden urge to scratch things. Good news: you can build a working hardness kit from a junk drawer in about ten minutes. A store-bought kit only earns its place once you need the finer steps at the hard end of the scale.

What comes in a Mohs hardness test kit?

Most kits you will see fall into two families. Both rely on the same rule from the Mohs hardness test: the harder material scratches the softer one, and you test both ways.

Reference mineral sets

These are small chunks of the real scale minerals. One typical boxed set, sold by the Geology.com store, holds nine specimens: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), feldspar (6), quartz (7), topaz (8) and corundum (9). Diamond is not included.

You scratch your mystery rock with each mineral in turn, then try the reverse. They are great for learning, because you handle the actual scale. The catch is that you need a clean, sharp edge on each piece, and the soft ones (talc and gypsum) wear down quickly.

Hardness pick sets

Hardness picks are pointed testers with tips of known hardness. The same store describes its pick set as four double-ended picks with eight points for Mohs 2 through 9: the 2 is plastic, the 3 is copper, and the others are alloys chosen to match the reference minerals. Its set also includes a grinding plate for sharpening the tips, a streak plate, a glass plate, small metal plates at 2.5 and 3.5, and a magnet.

The big advantage is precision. A fine point lets you scratch one small grain in a rock, or a tiny crystal, where a chunk of topaz would be clumsy.

How do you build a DIY hardness kit at home?

The Mineralogical Society of America's Mineral Identification Key has a do-it-yourself kit list, and much of it lives in an ordinary house. Here is a household version, with hardness values from the MSA key and the University of Auckland.

The MSA list goes further, and the extras are worth copying. A fairly strong fridge magnet checks for magnetite. A 10x hand lens lets you tell a groove from a smear. Labels and a pen stop your finds from turning into mystery rocks all over again. Keep it all together in a small box or pouch so you always know where it is.

For a quick fizz check on calcite, a dropper bottle of white vinegar is the gentle home version of the dilute acid geologists use; our vinegar fizz test guide explains it. And the tile does double duty: turn it over to run a streak test.

How do you use a hardness kit?

Start in the middle of the scale, not at the bottom. Try the steel nail first:

1. The nail scratches it. Move down: try copper, then your fingernail, until something fails.
2. The nail does not scratch it. Move up: see whether your rock scratches glass, then whether the file or a quartz piece scratches it.
3. Stop when you find the pair that brackets it, such as "scratched by the file but scratches glass," which puts it around 6.

Always test both ways and rub every mark with a wet finger. A line that wipes away is powder from the softer material, not a scratch. The MSA key warns that some samples take several tries before the result is clear, so repeat on a second spot.

When is a store-bought kit worth it?

A household kit tells quartz from calcite and feldspar from fluorite, which covers a lot of backyard and river finds. A bought kit starts to make sense when:

• You keep landing near the top. Household testers stop around the file and the tile. To tell quartz (7) from beryl (7.5 to 8) or topaz (8), you need quartz, topaz and corundum pieces. The MSA kit list includes exactly those, plus fluorite.
• Your specimens are small. Crystals in a vug (a small cavity in a rock) or single grains need the fine point of a pick.
• You teach. A set of the real reference minerals makes the scale click for kids and classes.
• Your testers are a mystery. Picks and mineral sets come with known values, so you are not guessing about a blade.

If you only want the hard end, you do not need a full set. Following the MSA list, a piece each of fluorite, quartz, beryl or topaz, and corundum fills every gap a kitchen drawer leaves. For the gems at that end of the scale, see our gemstone hardness chart.

The RockhoundID app, coming soon, will suggest likely matches from a photo and tell you which scratch test would confirm them, so you know which tester to reach for.

The takeaway

You do not need to buy anything to start testing hardness. A fingernail, an old penny, a steel nail, a piece of glass, a file and the back of a tile cover the scale up to about 7. Add a magnet, a hand lens and labels, keep it all in one box, and calibrate your steel. When your finds keep landing near the top of the scale, or your specimens get tiny, that is the moment to add a few reference minerals or a set of hardness picks.

The pictures in this guide are AI-generated illustrations of what to look for; real specimens vary, so trust the tests over any picture. RockhoundID guides help you narrow down what you found. They suggest likely candidates, not certain identifications or appraisals. For anything valuable, rare or possibly from space, ask a museum, a university geology department or your state geological survey, and check the collecting rules for the land before you take anything home.