What is the Mohs Scale and Why it’s Useful in Jewellery Making.

Ever wondered how to determine if a stone is hard or soft without smashing it to bits?

In jewellery making, we handle a variety of stones, some incredibly delicate and others nearly indestructible. But how do we know a stone's hardness without risking damage? Enter the Mohs scale, a system used by geologists (and jewellers!) to measure the hardness of minerals. Developed in 1812 by German mineralogist Friedrich Mohs, this scale helps compare minerals based on their ability to scratch one another.

The Mohs scale is simple: minerals are ranked from 1 (the softest) to 10 (the hardest). Mohs selected ten reference minerals to illustrate this, with talc at the bottom and diamond at the top:

1. Talc (1)

2. Amber (2)

3. Calcite (3)

4. Fluorite (4)

5. Apatite (5)

6. Opal (6)

7. Quartz (7)

8. Topaz (8)

9. Ruby (9)

10. Diamond (10)

How the Mohs Scale Works?

It is worth remembering that the Mohs scale is not an exact measurement. However, as jewellers, it is an essential tool that helps select the right tools to cut and polish gemstones, and guides us how to set and protect them.

The basic principle is straightforward: if one material can scratch another, it is harder. This means a softer stone requires gentler handling and protective settings to avoid damage.

Why the Mohs Scale Matters?

Knowing a stone's hardness is crucial when determining how durable it will be in a piece of jewellery. Harder stones like diamond, ruby, and quartz are more resistant to scratches and are ideal for everyday wear. Softer stones like opal, turquoise, and pearl are more susceptible to damage and better suited for occasional pieces.

For jewellers, understanding hardness also influences the cutting, shaping, and polishing of stones. It can even help us give clients advice on how to care for their jewellery.

7 Common Stones and Their Hardness Ranking

Now that we’ve covered the basics, let’s explore five popular stones in jewellery making. We’ll examine their properties and how their hardness affects their use:

1. Opal (Mohs 5.5–6.5)

• Properties:
  Opal is a hydrated form of silica, known for its stunning play of colour (opalescence), which makes it highly prized in jewellery. It can range from milky white to fiery red, with iridescent flashes of green, blue, and yellow.

• Hardness Impact:
  Opal is relatively soft, scoring between 5.5 and 6.5 on the Mohs scale. Its internal structure contains a high water content, making it more prone to cracking or chipping over time, especially when exposed to heat or pressure. For jewellers, this means that opal requires careful handling and protective settings, like bezels, to prevent damage. 

• Practical Uses:
  Opals are most often used in pendants, earrings, or brooches —pieces that are less likely to face impact. They need protective bezels and I suggest a gentle rubbing over the stone to prevent chipping it. If you are just starting your setting stone journey, then I would recommend harder gemstones like quartz or ruby (check out my courses for more info).
  

2. Tourmaline (Mohs 7–7.5)

• Properties:
  Tourmaline is a complex boron silicate mineral available in a wide range of colours, from deep green and pink to bi-coloured varieties like watermelon tourmaline.

• Hardness Impact:
  With a hardness of 7 to 7.5, tourmaline is durable enough for daily wear but can still be scratched by harder gemstones like topaz or diamond. Jewellers appreciate tourmaline for its versatility and vibrant hues, but care should be taken to avoid rough handling or exposure to harder stones.

• Practical Uses:
  Tourmaline is often used in rings, necklaces, and bracelets due to its durability. However, it is a challenging gemstone as it can easily be scratched while handled.
  

3. Quartz (Mohs 7) – Amethyst and Citrine

• Properties:
  Quartz is a common and versatile mineral, available in many forms such as amethyst, citrine, and rose quartz. Quartz is one of the most abundant minerals on Earth.

• Hardness Impact:
  Quartz, with a hardness of 7, is durable enough for everyday wear but can still be scratched by harder stones like topaz or diamond. It offers a great balance between beauty and practicality.

• Practical Uses:
  Amethyst and citrine are popular in a variety of jewellery pieces, including rings, pendants, and earrings. Their durability makes them suitable for everyday wear and they are perfect to set for beginners.
  

4. Emerald (Mohs 7.5–8)

• Properties:
  Emerald is a green variety of the mineral beryl, coloured by trace amounts of chromium or vanadium. It is known for its rich, vibrant green hue.

• Hardness Impact:
  While it's fairly hard, its structure often includes internal fractures (called "inclusions"), making it more fragile than its hardness would suggest. Jewellers need to be cautious when cutting or setting emeralds to avoid chipping or breakage.

• Practical Uses:
  Emeralds are often used in rings, pendants, and bracelets but require protective settings to shield them from impact. To prevent damage, it has been invented the emerald cut, which is typical of this stone. It is characterised by a rectangular shape with step-like, parallel facets.
  

5. Topaz (Mohs 8)

• Properties:
  Topaz is a silicate mineral that can be found in a range of colours, including blue, yellow, pink, and clear. It is prized for its clarity and brilliance.

• Hardness Impact:
  Topaz scores an 8 on the Mohs scale, making it a hard gemstone but still more prone to chipping or cleaving due to its crystal structure. I suggest to handle them with care during the cutting and setting process to avoid unnecessary damage.

• Practical Uses:
  Topaz is used in rings, necklaces, and bracelets, and while it is durable, jewellers need to be mindful of its tendency to chip under sharp blows. It’s recommended to set topaz in protective designs like bezels.
  

6. Ruby and Sapphire (Mohs 9)

• Properties:
  Both ruby and sapphire are varieties of corundum, with ruby being red and sapphire typically blue (though sapphires come in various colours).

• Hardness Impact:
  With a hardness of 9, rubies and sapphires are extremely durable and highly resistant to scratching. This makes them excellent choices for everyday jewellery.

• Practical Uses:
  Given their hardness, rubies and sapphires are perfect for any jewellery type, including rings, earrings, and bracelets. Their hardness allows for intricate cuts and settings, providing jewellers with creative flexibility, though they are expensive to source.
  

7. Diamond (Mohs 10)
Properties:
Diamond is the hardest natural substance known, with a perfect score of 10 on the Mohs scale. Its unparalleled hardness and brilliance make it one of the most prized gemstones.

Hardness Impact:
Diamonds are virtually unscratchable, which allows jewellers to experiment with various cuts and settings without concern for damaging the stone. However, they can still chip if struck with enough force, so care must still be taken.

Practical Uses:
Diamonds are commonly used in engagement rings and high-end jewellery. Their hardness ensures they retain their brilliance even with frequent wear. Despite their toughness, they should still be handled with care to avoid chipping.

Conclusion

For jewellers, the Mohs scale is an essential tool in understanding gemstone hardness and durability. It helps us choose the right materials and techniques to craft beautiful, long-lasting pieces. However, it's important to remember that hardness is not the same as toughness. Even the hardest stones, like diamonds, can crack if subjected to the wrong type of pressure.

By knowing a gemstone’s hardness, we can make informed decisions about its care, wearability, and overall use, ensuring that each piece of jewellery maintains its beauty and stands the test of time.