The Mohs Scale of Mineral Hardness, developed in 1812 by German mineralogist Friedrich Mohs, provides a relative ranking of minerals based on their ability to resist scratching. The scale ranges from 1 (talc, the softest) to 10 (diamond, the hardest). Unlike other scientific hardness tests that measure absolute hardness, the Mohs scale is comparative—each mineral can scratch those with lower numbers and will be scratched by those with higher numbers. This practical approach helps jewelers and gemologists determine how different gemstones will withstand everyday wear.
The Mohs scale uses ten reference minerals: 1-Talc, 2-Gypsum, 3-Calcite, 4-Fluorite, 5-Apatite, 6-Orthoclase Feldspar, 7-Quartz, 8-Topaz, 9-Corundum, and 10-Diamond. The scale is not linear in terms of absolute hardness—the jump from 9 to 10 represents a much larger increase in hardness than the jump from 1 to 2. In fact, diamonds (10) are approximately four times harder than corundum (9), which includes rubies and sapphires, while the difference between corundum and topaz (8) is less dramatic.
When selecting gemstones for jewelry that will be worn daily, such as engagement rings, hardness is a crucial consideration. Gemstones with a Mohs rating of 7 or higher are generally recommended for everyday wear. Diamonds (10), rubies and sapphires (9), and topaz (8) all excel in this category. Quartz varieties like amethyst and citrine (7) are also durable choices. Emeralds, despite their popularity, rate lower (7.5-8) and contain natural inclusions that can make them more vulnerable to damage. Softer stones like opal (5.5-6.5), tanzanite (6-7), and pearl (2.5-4.5) are better suited for occasional wear or in protected settings.
While the Mohs scale measures scratch resistance, it doesn't account for other durability factors like toughness (resistance to breaking or chipping) and stability (resistance to chemical or environmental damage). Jade, for example, has a moderate hardness of 6-7 but exceptional toughness, making it remarkably durable. Conversely, diamond, despite being the hardest natural substance, has perfect cleavage planes that can make it vulnerable to chipping if struck at certain angles. Some gemstones, like opals or pearls, may be sensitive to temperature changes, household chemicals, or prolonged sunlight exposure, regardless of their hardness rating.
Understanding the Mohs scale helps collectors and jewelry enthusiasts make informed decisions. For instance, knowing that a topaz (8) can scratch an amethyst (7) informs proper storage practices to prevent damage. The scale also explains why certain gemstones are more frequently found with surface abrasions—softer stones like malachite (3.5-4) or fluorite (4) require more protective settings and careful handling. For custom jewelry designs, the hardness rating helps determine appropriate uses: harder stones for rings and bracelets that face frequent impacts, and softer stones for earrings and pendants that encounter less abrasion.