Hardness (H) of minerals is identified with the Mohs scale of hardness. Created by a German mineralogist named Friedrich Mohs in 1812, the Mohs scale places minerals in a degree of hardness based on the difficulty of which the mineral can be scratched by another mineral or steel tool.
Hardness plays an important role in jewelry making since only the hardest stones can be faceted – meaning only they can be cut into shapes to reflect even more light and sparkle. Examples of these stones include diamond, topaz, ruby, and sapphire.
Softer stones are often more simply smoothed and polished for jewelry as they will shatter if trying to cut them into a shape.
The Mohs Scale
Several common objects can be used to help identify the hardness of a mineral including a fingernail, steel knife, glass, or a copper coin – to name a few.
Hardness and Chemical Composition
There is a direct correlation between hardness and chemical composition which allows for some overall generalizations to be made with minerals.
- Most sulfides are relatively soft (H < 5). The exceptions to this would be pyrite and marcasite, which hold a hardness of 6 – 6.5.
- Most hydrous minerals are soft (H < 5).
- Sulfates are generally soft (H < 5.5).
- Carbonates are relatively soft (H < 5.5).
- Phosphates are relatively soft (H < 5.5).
- Halides are relatively soft (H < 5.5).
- Most anhydrous silicates and oxides are relatively hard (H > 5.5).
If you are beginning a journey into jewelry making, then understanding the hardness of minerals will be one of the most important concepts to understand and will save you a ton of time and grief when cutting or polishing certain stones.