Understanding Mineral Hardness

First established in 1812 by German geologist Friedrich Mohs, the Mohs Hardness Scale is a qualitative ordinal scale ranking minerals from 1 (softest) to 10 (hardest).

For geologists in the field and lapidarists in the workshop, understanding mineral hardness is essential for identification and processing.

This Mohs scratch test guide outlines how to safely determine a mineral's hardness rating using simple tools.

The Mohs Scale Reference Values

Below is the standard reference scale of minerals, along with common household testing tools:

| Mohs Value | Reference Mineral | Common Testing Tool | Tool Hardness Value | | :---: | :--- | :--- | :---: | | 1 | Talc | Soapstone / Chalk | - | | 2 | Gypsum | Fingernail | 2.5 | | 3 | Calcite | Copper Penny / Wire | 3.0 | | 4 | Fluorite | Pocket Knife / Iron Nail | 4.5 | | 5 | Apatite | Glass Plate | 5.5 | | 6 | Orthoclase | Steel File / Masonry Drill | 6.5 | | 7 | Quartz | Hardened Steel | 7.0 - 8.0 | | 8 | Topaz | Quartz Crystal | - | | 9 | Corundum (Ruby/Sapphire)| Silicon Carbide (Carborundum) | 9.5 | | 10 | Diamond | Diamond Grit | 10.0 |

Step-by-Step Scratch Testing Procedure

To get an accurate hardness reading, follow these steps:

Step 1: Select a Testing Site

Find a flat, unexposed surface on the test mineral. Wipe away any loose dirt, rust, or grit, as these can give false readings.

Step 2: Attempt the Scratch

  1. Start with the softest testing tool, such as your fingernail (2.5) or a copper penny (3.0).
  2. Press the edge of the tool firmly against the mineral's surface and draw it across.
  3. Crucial: Wipe away any dust generated. Rub your finger over the line. Is it a true gouge (scratch), or did the testing tool simply rub off onto the harder mineral (powder streak)?

Step 3: Bracket the Hardness

  • If your fingernail (2.5) cannot scratch the stone, but a copper penny (3.0) can, the mineral's hardness is approximately 2.5 to 3.0 (likely Calcite).
  • Repeat the test with progressively harder tools until the mineral is scratched, narrowing down its Mohs value.

For more scientific details and research on this topic, you can check out the Geology.com.