---
title: "Moh's scale of hardness"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:mohs-scale-of-hardness"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/mohs-scale-of-hardness
site: BCT Engineering
---

# Moh's scale of hardness

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2081 Bhadra, 1 marks; 2081 Baisakh, 1 marks; 2080 Baisakh, 1 marks; 2078 Bhadra, 1.5 marks; 2078 Kartik, 1.5 marks; 2076 Chaitra, 1 marks; 2076 Asoj, 1 marks; 2075 Chaitra, 1.5 marks; 2075 Asoj, 1 marks; 2072 Chaitra, 1 marks; 2069 Chaitra, 1.5 marks; 2066 Bhadra (old course), 2 marks; 2066 Jestha (old course), 4 marks; 2062 Baisakh (old course), 2 marks (asked 14 times)
- Video: [What is Mohs' HARDNESS Scale?](https://www.youtube.com/watch?v=WCcn_FvFM4s) — HighSchoolScience101
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/mohs-scale-of-hardness
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Define hardness of minerals. Describe Moh's scale of hardness.

## Short answer

Hardness is a mineral’s resistance to scratching. Mohs scale arranges ten minerals from talc (1) to diamond (10), each scratching those below it. Common field indicators: fingernail (~2.5), copper coin (~3.5), knife/glass (~5.5), steel file (~6.5).

## Answer

**Hardness** of a mineral is its resistance to scratching (abrasion). It depends on the strength of the chemical bonds and the atomic structure of the mineral.

#### Mohs scale
In 1812 Friedrich Mohs proposed a relative scale of ten standard minerals, in which each mineral scratches those below it.

| Hardness | Mineral | Simple test |
|---|---|---|
| 1 | Talc | Scratched by fingernail, soapy feel |
| 2 | Gypsum | Scratched by fingernail |
| 3 | Calcite | Scratched by copper coin |
| 4 | Fluorite | Scratched by knife easily |
| 5 | Apatite | Scratched by knife with difficulty |
| 6 | Orthoclase feldspar | Scratches glass with difficulty; scratched by steel file |
| 7 | Quartz | Scratches glass easily |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum | Scratches topaz |
| 10 | Diamond | Hardest; scratches all |

#### Field aids
- Fingernail: about 2.5
- Copper coin: about 3.5
- Knife blade / glass: about 5.5
- Steel file: about 6.5

#### Notes
- It is a **relative, ordinal** scale: the gap between 9 and 10 is much larger than between 1 and 2.
- To test an unknown mineral, find which standard mineral or object scratches it and which it scratches; its hardness lies between them.
- Test a fresh surface and not a weathered or powdery one.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Hardness of a mineral is its resistance to:
   A. Scratching (abrasion)
   B. Splitting along planes
   C. Breaking under a load
   D. Dissolving in acid
   Answer: A. Hardness is scratch resistance; splitting is cleavage. (Key point: Definition)

2. Which is the correct order of Mohs' scale from 1 upward (first five)?
   A. Talc, calcite, gypsum, fluorite, apatite
   B. Talc, gypsum, calcite, fluorite, apatite
   C. Gypsum, talc, calcite, apatite, fluorite
   D. Talc, gypsum, fluorite, calcite, apatite
   Answer: B. The scale is talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5. (Key point: Scale order)

3. Why is Mohs' scale called relative (ordinal)?
   A. Each step is exactly twice as hard as the previous
   B. It is based on measured force in MPa
   C. It applies only to rocks, not minerals
   D. The gaps between steps are unequal; for example 9 to 10 is much larger than 1 to 2
   Answer: D. It only ranks scratch ability; it does not give equal intervals. (Key point: Nature of scale)

4. How do you find the hardness of an unknown mineral?
   A. Weigh it and compare with water
   B. Observe its streak on porcelain
   C. Find which standard mineral or object scratches it and which it scratches; the hardness lies between
   D. Test its reaction with dilute HCl
   Answer: C. Scratching against references brackets the hardness. (Key point: Testing an unknown)

## Related in Engineering Geology I

- [Tenacity and hardness of minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atenacity-and-hardness-of-minerals) — Engineering Geology I, chapter 3