Understanding the Mohs Hardness Scale

## What the Mohs Scale Actually Measures Friedrich Mohs, a German mineralogist, developed his hardness scale in 1812. He took ten minerals and arranged them in order: each mineral in the sequence can scratch all minerals below it and can be scratched by all minerals above it. The scale runs from 1 (talc) to 10 (diamond). The Mohs scale measures scratch resistance, not brittleness, toughness, or absolute hardness. This distinction matters. Diamond is a 10 on Mohs — no other mineral can scratch it — but diamond can be cleaved or shattered by a targeted blow because it has perfect cleavage in four directions. A jeweler who strikes a diamond at the wrong angle with a hammer can split it cleanly. The scale is also not linear. The absolute hardness difference between 9 (corundum/sapphire) and 10 (diamond) is far larger than the difference between any other adjacent steps. Diamond is roughly 4 times harder than sapphire by absolute measurement, even though they are only one step apart on the Mohs scale. ## The Scale | Mohs | Mineral | Common Example | |------|---------|----------------| | 1 | Talc | Talcum powder | | 2 | Gypsum | Alabaster, selenite | | 3 | Calcite | Marble, limestone | | 4 | Fluorite | Fluorite (gem quality) | | 5 | Apatite | Some brown/yellow gems | | 6 | Orthoclase | Moonstone (6–6.5) | | 7 | Quartz | Amethyst, citrine, jasper | | 8 | Topaz | Blue topaz, imperial topaz | | 9 | Corundum | Ruby, sapphire | | 10 | Diamond | Diamond | Household reference points: a fingernail is approximately 2.5, a copper coin is 3.5, a steel nail is approximately 5.5–6, and window glass is about 5.5. ## Gemstone Hardness Values Fine gemstones span nearly the entire Mohs range: **Hardness 10**: Diamond (3.51 SG, pure carbon). The hardest natural substance. Suitable for all jewelry forms including rings worn daily. **Hardness 9**: Ruby, Sapphire (both corundum, SG 4.0). Excellent for rings and everyday wear. Only diamond and other corundum can scratch them. **Hardness 8–8.5**: Spinel (8), Topaz (8), Alexandrite/Chrysoberyl (8.5), Aquamarine/Emerald (7.5–8 beryl family). Good ring stones with proper care. **Hardness 7–7.5**: Quartz family (amethyst, citrine, ametrine, rose quartz at 7), Tourmaline (7–7.5), Garnet (6.5–7.5 depending on variety), Zircon (6.5–7.5), Tanzanite (6.5–7). Suitable for rings but will show wear over time as they are at or near the hardness of common dust (which contains quartz). **Hardness 6–7**: Peridot (6.5–7), Moonstone (6–6.5), Opal (5.5–6.5). Better suited to earrings, pendants, and protected ring settings. Opal in particular is fragile — it contains water and can craze (develop surface cracks) if exposed to temperature extremes. **Hardness below 6**: Pearl (2.5–4.5), Coral (3–4), Amber (2–2.5), Jet (2.5–4). These organic gems require careful handling. Never clean pearl in ultrasonic cleaners or with harsh chemicals. Hairspray, perfume, and cosmetics degrade pearl nacre over time. ## Why Hardness Matters in Practice **Ring wear**: Stones set in rings that contact surfaces frequently (an engagement ring, for example) should ideally be Mohs 7 or higher. Household dust contains quartz particles (Mohs 7), which will abrade the surface of any stone softer than 7, gradually dulling its polish. This is why moonstone and opal engagement rings require more maintenance than sapphire or diamond rings. **Identification**: Hardness testing is a non-destructive identification method when done carefully. Scratching an inconspicuous area of a stone with a material of known hardness gives a data point. However, this is only done on rough or in situations where a small mark is acceptable — never on faceted gems in fine settings. **Storage**: Stones should be stored separately to avoid harder stones scratching softer ones. Diamonds will scratch sapphires; sapphires will scratch garnets; garnets will scratch opals. Compartmentalized jewelry boxes or individual pouches prevent contact. **Hardness vs. Toughness**: Jade is only 6.5–7 on Mohs but is famously tough. Its interlocking crystalline structure (jadeite) or fibrous structure (nephrite) resists fracture even under significant impact. Conversely, fluorite is 4 on Mohs and also brittle — a double durability disadvantage. Hardness and toughness together determine a stone's practical durability for jewelry use.