Physical Properties
Hardness, specific gravity, cleavage, fracture, and lustre.
Introduction
Physical properties are essential for gem identification and determining
suitability for different types of jewellery. These properties depend on
the crystal structure and chemical composition.
Hardness
Hardness measures a mineral's resistance to scratching. The Mohs scale
ranks minerals from 1 (talc) to 10 (diamond), but the scale is not linear—diamond
is approximately 140 times harder than corundum.
| Hardness | Mineral Standard | Common Gemstones |
|---|---|---|
| 10 | Diamond | Diamond |
| 9 | Corundum | Ruby, Sapphire |
| 8 | Topaz | Topaz, Spinel, Chrysoberyl |
| 7 | Quartz | Amethyst, Citrine, Tourmaline |
| 6-7 | Feldspar | Peridot, Jadeite |
| 5-6 | Apatite | Opal, Turquoise |
Hardness Considerations
Specific Gravity (SG)
Specific gravity is the ratio of a gem's density to that of water. It's a
diagnostic property that can be measured without damaging the stone using
hydrostatic weighing.
Formula: SG = Weight in air / (Weight in air - Weight in water)
| Range | Gemstones |
|---|---|
| 2.0-2.5 | Opal (2.15), Amber (1.08) |
| 2.5-3.0 | Quartz (2.65), Feldspar (2.56-2.76) |
| 3.0-3.5 | Tourmaline (3.06), Jadeite (3.34) |
| 3.5-4.0 | Diamond (3.52), Topaz (3.53), Spinel (3.60) |
| 4.0-4.5 | Corundum (4.00), Zircon (4.69) |
| >5.0 | High zircon (4.69), Cassiterite (7.0) |
Cleavage
Cleavage is the tendency of a mineral to break along flat planes related to
its crystal structure. These planes are directions of weaker atomic bonding.
| Quality | Description | Examples |
|---|---|---|
| Perfect | Very smooth, flat surfaces | Topaz {001}, Fluorite {111} |
| Good | Relatively flat but may be stepped | Feldspar, Spodumene |
| Distinct | Recognizable but not dominant | Beryl (imperfect) |
| Poor/None | No preferred break direction | Quartz, Garnet |
Cleavage Hazards
Fracture
Fracture describes how a mineral breaks in directions other than cleavage planes.
| Type | Appearance | Examples |
|---|---|---|
| Conchoidal | Curved, shell-like surfaces | Quartz, Glass, Obsidian |
| Uneven | Rough, irregular surface | Jadeite |
| Splintery | Fibrous or needle-like | Nephrite |
| Hackly | Jagged, sharp edges | Native metals |
Lustre
Lustre describes how light interacts with a mineral's surface. It depends on
the refractive index and surface quality.
| Lustre | Description | Examples |
|---|---|---|
| Adamantine | Brilliant, diamond-like (RI >1.9) | Diamond, Zircon |
| Vitreous | Glass-like (most common) | Quartz, Beryl, Tourmaline |
| Resinous | Like resin or plastic | Amber, Sphalerite |
| Waxy | Like candle wax | Turquoise, Chalcedony |
| Pearly | Iridescent, like pearl | Moonstone, some feldspar |
| Silky | Like silk fabric | Tiger's eye, Satin spar |
Tenacity
Tenacity describes a mineral's resistance to breaking, bending, or crushing:
- Brittle - Shatters when struck (most gemstones)
- Tough - Resists breaking despite lower hardness (nephrite, jadeite)
- Sectile - Can be cut with a knife
- Flexible - Bends but doesn't return (mica)
- Elastic - Bends and returns to shape
Toughness vs Hardness
Specific Gravity Measurement Techniques
Specific gravity (SG) is one of the most reliable diagnostic properties and can be
measured without damaging the stone. Two main methods are used in gemmology.
Hydrostatic Weighing
The most accurate method for loose stones:
- Weigh stone in air (W₁)
- Suspend stone in water on a wire bridge
- Weigh stone in water (W₂)
- Calculate: SG = W₁ / (W₁ - W₂)
Accuracy considerations:
- Use distilled water at 4°C for best accuracy
- Account for surface tension on small stones
- Wire bridge weight must be tared
- Ensure no air bubbles trapped on stone
- Multiple measurements improve reliability
Heavy Liquids Method
Liquids of known SG can quickly estimate stone density:
- Methylene iodide (SG 3.32): Good for many gems
- Clerici solution (SG up to 4.2): For denser stones
- Diluted solutions: Create intermediate densities
If stone floats, SG < liquid; if **sinks**, SG > liquid; if suspends, SG = liquid
Note: Many heavy liquids are toxic; follow safety protocols.
SG Accuracy and Limitations
| Factor | Effect on SG |
|---|---|
| Inclusions | May raise or lower SG depending on inclusion type |
| Fractures | May trap air, lowering apparent SG |
| Porosity | Porous stones absorb water, affecting result |
| Temperature | Water density changes with temperature |
| Stone size | Very small stones harder to measure accurately |
Directional Hardness
Some minerals have significantly different hardness depending on crystal direction.
This property, called directional hardness or anisotropic hardness, has important
practical implications.
Kyanite
Kyanite is the classic example of extreme directional hardness:
- Parallel to c-axis (length): H 4-4.5
- Perpendicular to c-axis: H 6-7
This 3-point variation affects cutting and polishing—different directions
require different approaches.
Diamond
Diamond's hardness varies by crystal direction:
- Octahedral faces {111}: Hardest direction
- Cube faces {100}: Slightly softer
- Dodecahedral faces {110}: Softest
Diamond cutters exploit these differences. Sawing is done parallel to
cubic planes; polishing uses the harder octahedral direction.
Practical Implications
Directional hardness affects:
- Polishing quality (some directions polish better)
- Wear patterns (soft directions wear faster)
- Cutting approach (orient for best finish)
- Durability assessment (softest direction matters)
Thermal Conductivity
Thermal conductivity measures how quickly heat flows through a material. This
property is particularly useful for diamond testing and identification.
Diamond vs Simulants
| Material | Thermal Conductivity | Thermal Probe Result |
|---|---|---|
| Diamond | Very high (2000+ W/m·K) | Positive (conducts heat rapidly) |
| Moissanite | High (490 W/m·K) | Positive (may test as diamond) |
| CZ | Low (2 W/m·K) | Negative |
| Glass | Very low (1 W/m·K) | Negative |
| Other gems | Low to moderate | Negative |
Thermal Probe Testing
Thermal testers work by measuring heat dissipation from a heated probe tip:
- Probe touches stone surface
- Heat flows from probe into stone
- Faster cooling = higher thermal conductivity
Limitations:
- Moissanite also conducts heat well (false positive)
- Metal mountings can affect readings
- Stone must be room temperature
- Small stones may give unreliable results
Combined Thermal-Electrical Testing
Modern testers combine thermal and electrical conductivity:
- Diamond: High thermal, no electrical conductivity
- Moissanite: High thermal, electrical conductivity
- This distinguishes diamond from moissanite reliably
Most synthetic moissanite (SiC) is an electrical semiconductor, while
natural diamond (except rare Type IIb blue) is an insulator.