Physical properties
Goal: Use hardness, toughness, cleavage and specific gravity correctly.
Mohs hardness (scratch resistance)
Mohs is a ranking, not a linear scale: diamond (10) is far harder than corundum (9). Dust (quartz, H7) dulls anything softer than 7, which is why H7+ matters for rings.
Toughness
Resistance to breaking. Nephrite and jadeite are the toughest gems; diamond is hard but can cleave.
Cleavage
Breaks along flat atomic planes. Topaz (basal), diamond (octahedral, 4 directions), fluorite, spodumene, feldspars.
Parting
Breaks along twin planes or exsolution layers. Corundum often parts.
Fracture
Irregular break. Conchoidal (shell-like) in quartz, glass, garnet.
Stability
Heat, light, chemicals. Kunzite fades in sun, opal crazes, pearls hate acid and perfume.
Do not scratch-test
Hardness tests damage gems. Use RI/SG; leave scratching to rough offcuts.
Specific gravity (SG) — the second big number
Worked example
Air 4.00 ct, water 3.00 ct → 4.00 ÷ 1.00 = 4.00 → corundum range (3.97–4.05).
Air 5.20 ct, water 3.25 ct → 5.20 ÷ 1.95 = 2.67 → beryl or quartz range.
Rules for a good reading
- Stone ≥ 1 ct for accuracy on a 0.01 ct scale
- Remove bubbles with a brush; use a drop of detergent
- Unmounted only; no porous stones (opal, turquoise)
- Repeat 3× and average
SG landmarks to memorise
Lustre
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