Using hardness pencils/raw materials is a quick and classic method for rough grading and identifying gemstones/minerals. It is based on the Mohs hardness scale, which tests a stone's ability to scratch or be scratched by reference materials. Since each mineral has a specific hardness, this is a very useful exclusion tool.
Here is a quick guide to how the pencils work:
Mohs Hardness Scale (Reference Material)
The scale ranges from 1 to 10, with 1 being the softest and 10 being the hardest (diamond). The hardness pencils/raw materials have tips made of different materials that correspond to the steps on the scale:
- 1–3: Talc, gypsum, calcite
- 4–5: Fluorite, apatite
- 6–7: Feldspar, quartz
- 8–10: Topaz, corundum (sapphire/ruby), diamond
How the test is done
- You choose a hardness on the pen and make a very careful, small scratch on a hidden part of the gemstone (for example, the underside or edge of the stone).
- If the stone is scratched: The stone is softer than the tip of the pen.
- If the stone is NOT scratched: The stone is as hard as or harder than the tip of the pen.
- If you are using raw material, the test is performed in the same way as the mineral to be tested.
Benefits and uses
- Fast and direct indication: Provides immediate answers in the field.
- Distinguishes fakes from genuine: Perfect for, for example, distinguishing genuine harder stones (such as sapphire with a hardness of 9) from softer imitations (such as glass with a hardness of 5-6).
Important limitations and risks
- Risk of damage: The method is destructive because it causes a permanent (albeit often small) scratch. This can destroy the value of cut and faceted collector stones.
- Brittle stones: A hard stone can be brittle and risk cracking if you press too hard.
- Crystal direction: Some stones have different hardnesses depending on which crystal axis you are testing (e.g. kyanite varies between 4 and 7).
- Safety margin: You should always start with a pencil with a lower hardness than you suspect the stone has to avoid damaging it unnecessarily.
