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
High density liquids can quickly estimate stone density by the float/sink/suspend
method. Two organic liquids commonly used for gemstone separation are
di-iodomethane and 1-bromonaphthalene, which can be mixed to give a useful
series of three standard SG values:
- 1-bromonaphthalene (SG 1.49): Also known as monobromonaphthalene
- Bromoform (SG 2.89): Useful intermediate density liquid
- Di-iodomethane (SG 3.33): Also known as methylene iodide; undiluted for densest standard
These liquids are diluted with each other to create intermediate densities.
The three SGs typically used for gem testing are approximately 2.65, 3.05, and 3.33.
If a stone floats, its SG is less than the liquid; if it sinks, its SG is
greater; if it suspends, its SG equals the liquid.
Note: Many heavy liquids are toxic and restricted. Di-iodomethane discolours
during storage and should be stored in a dark bottle. Heavy liquid SG will vary
slightly with temperature and differential evaporation – values should be taken
as approximate.
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. The mineral was
once commonly called "disthene" meaning "two strengths":
- Parallel to length (c-axis): H 4.5-5
- Perpendicular to length: H 6.5-7
This difference in hardness means the stone is difficult to cut and must be
handled with care.
Jadeite
Older pieces of jadeite polished with softer materials may exhibit an "orange peel"
surface – an irregular polish on a fine scale caused by the polycrystalline
structure, in which randomly oriented grains each present different hardness
directions to the polishing powder. Diamond powder gives an even polish in all
directions regardless of this effect. A similar undercutting effect is also seen
in lapis lazuli and bloodstone.
Diamond
Diamond's hardness varies by crystal direction:
- Hardest: Diagonal directions on cube planes
- Hard: All directions on octahedral planes
- Softest: Across dodecahedral planes
Only diamond can cut diamond, and this is possible solely because of differential
hardness. Diamond grit or powder used to cut and polish diamonds is a mass of
particles presenting variously oriented directions of maximum hardness. There is
always a large number of particles correctly aligned to abrade the softer directions
of the diamond being sawn or polished.
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)
Streak Test
Some minerals and ornamental materials give a characteristically coloured powder
when scratched, or when drawn across an unglazed white porcelain plate. Streak tests
are most easily carried out on unpolished or rough material, and the colour of the
streak on the white plate may provide good supplementary evidence of identity.
Like hardness testing, it is a destructive test and seldom used for general
gemmological testing.
| Material | Body Colour | Streak Colour |
|---|---|---|
| Hematite | Metallic grey/black | Red to reddish-brown |
| Gold | Metallic gold | Metallic gold |
| Pyrite (fool's gold) | Metallic gold | Greenish-black |
| Malachite | Banded green | Pale green |
| Lapis lazuli | Rich blue | Blue |
| Sodalite | Deep blue | White |
| Whitby jet | Black | Dark chocolate-brown* |
Parting
Parting is a tendency of a material to split along twinning planes. It can look
similar to cleavage and is sometimes called "false cleavage".
Unlike cleavage, which is an inherent property of the crystal structure found
throughout a material, parting occurs only along specific planes of weakness –
typically twin boundaries – and is not found in all specimens of a species.
Parting in Corundum
Parting is common in corundum, where well-developed parting may be seen
parallel to rhombohedral faces. This occurs along twinning planes within
the crystal and is not present in all corundum specimens.
Parting can be mistaken for cleavage during examination, but true cleavage
in corundum does not exist – the rhombohedral "cleavage" observed is
actually parting along twin planes.
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.
Tenacity – Resistance to Mechanical Deformation
Tenacity describes how a mineral resists breaking, bending, or deformation. It is distinct
from hardness (resistance to scratching) and from cleavage (the mode of fracture). The
principal tenacity terms used in gemmology are:
- Brittle: fractures without plastic deformation; shatter on impact or under stress.
The fracture surface is conchoidal, uneven, or hackly. Essentially all silicates and
oxides in common gemmology are brittle. - Tough: high resistance to fracture despite lower hardness than brittle gems; typically
an interlocking aggregate structure disperses crack energy. Nephrite and jadeite are the
gemmological standards. - Sectile: can be cut with a knife into shavings without crumbling; deforms plastically
under a sharp edge. Not common in gem minerals; talc (H1) is sectile. - Malleable: can be hammered into thin sheets without crumbling. Metals only – native
gold inclusions in quartz are malleable. - Ductile: can be drawn into a wire. Native gold, platinum. Not a gemmological property
of the gem minerals themselves. - Flexible: thin plates or sheets can be bent without breaking but do not return to
shape (plastic deformation). Talc, chlorite, gypsum selenite plates. - Elastic: thin plates can be bent and spring back to original shape without permanent
deformation. Distinguishes muscovite mica (elastic) from chlorite (flexible but not elastic).
Source: Read, Gemmology 3rd ed. DOI: 10.4324/9780080507224 [VERIFIED]
Tenacity in gem identification
| Tenacity | Gem/mineral examples | Diagnostic application |
|---|---|---|
| Brittle | Diamond, corundum, quartz, tourmaline, topaz, garnet, spinel | Most faceted gems; conchoidal or uneven fracture surface |
| Tough | Nephrite (H 6–6.5), jadeite (H 6.5–7) | Nephrite tougher than diamond despite lower hardness – fibrous interlocking structure |
| Sectile | Amber (slightly under knife), talc | Amber can be shaved; relevant in amber vs plastic vs copal testing |
| Malleable | Native gold (inclusion in quartz) | Gold leaf inclusions spread under pressure |
| Flexible (not elastic) | Talc, chlorite, gypsum selenite | Chlorite inclusions in gems flex under probe but do not spring back |
| Elastic | Muscovite, phlogopite mica | Mica inclusions spring back when probe is removed – distinguishes from chlorite |
Toughness vs hardness
The toughness/hardness distinction is examinable at Diploma level:
- Hardness = resistance to scratching (Mohs scale, a surface property).
- Toughness = resistance to fracture (a bulk property of the interlocked structure).
Nephrite jade (H 6–6.5) is tougher than diamond (H 10) because its densely interlocking
fibrous tremolite–actinolite structure prevents crack propagation. Diamond, despite maximum
hardness, has perfect octahedral cleavage and brittle tenacity – a sharp blow in the {111}
direction will cleave it.
Magnetism in Gems
Gem minerals may be diamagnetic, paramagnetic, or ferromagnetic/ferrimagnetic. A strong N52
neodymium magnet provides a rapid, non-destructive field test that correlates with
iron (Fe) and manganese (Mn) content.
Magnetic behaviour types:
- Diamagnetic: all materials without unpaired electrons have a very weak repulsive
response to a magnetic field. Most gem silicates with no transition metal are effectively
diamagnetic (quartz, topaz, diamond, spinel). - Paramagnetic: materials with unpaired electrons in the 3d shell are weakly attracted.
Susceptibility follows the Curie law (χ ∝ 1/T). Fe²⁺ (4 unpaired electrons), Fe³⁺
(5 unpaired), Mn²⁺ (5 unpaired), Cr³⁺ (3 unpaired) all contribute. Gems with high Fe
or Mn content are detectably paramagnetic with a strong N52 magnet. - Ferrimagnetic: exchange-coupled spins; found in iron oxide inclusions (magnetite
Fe₃O₄) rather than in gem silicates themselves. A stone with sufficient magnetite
inclusions can be attracted even if the host is diamagnetic.
Source: Hoover, Williams, Williams & Mitchell, J. Gemmology, 2008.
DOI: 10.15506/jog.2008.31.3.91 [VERIFIED]; Hoover, Gems & Gemology, 2011.
DOI: 10.5741/gems.47.4.272 [VERIFIED]
Gem species magnetic response
| Gem species | Response | Reason |
|---|---|---|
| Almandine garnet (Fe-rich) | Strongly attracted | High Fe²⁺/Fe³⁺; largest susceptibility among garnets |
| Spessartine garnet (Mn-rich) | Attracted | Mn²⁺ (5 unpaired electrons); varies with Mn content |
| Demantoid garnet (andradite, Fe³⁺) | Attracted | Paramagnetic from Fe³⁺; useful vs green tourmaline |
| Tsavorite (grossular with V, Cr) | Weakly attracted | Cr and V paramagnetic; weaker than almandine |
| Pyrope (pure end-member) | Very weakly attracted or diamagnetic | Low Fe, low Mn; rhodolite (pyrope-almandine) intermediate |
| Peridot (Fe²⁺ in olivine) | Attracted | Fe²⁺ essential; separable from green tourmaline or green sapphire |
| Blue/indicolite tourmaline (Fe-bearing) | Weakly attracted | Fe-bearing but weaker response than Fe-rich garnets |
| Fe-bearing sapphire | Weakly attracted | Fe paramagnetic; less than garnets |
| Diamond, quartz, spinel, topaz | No response (diamagnetic) | No unpaired d-electrons in pure form |
| Magnetite inclusions (in star sapphire, star diopside etc.) | Strongly attracted | Ferrimagnetic Fe₃O₄; even if host is diamagnetic |
Diagnostic applications
- Rapid field separation: a N52 magnet separates almandine, spessartine, and demantoid
from diamagnetic simulants (glass, CZ) without instruments. - Garnet composition: Hoover (2008, 2011) demonstrated that magnetic susceptibility
measurement can resolve garnet species and compositions more precisely than RI alone,
especially for mixed-composition stones in the almandine–pyrope–spessartine series. - Above-refractometer garnets: demantoid RI (
1.89) exceeds the standard refractometer1.79). The magnet test complements by confirming the garnet identification.
range ( - Safety note: N52 neodymium magnets are extremely strong; handle with care around
electronic equipment, credit cards, and polishing compounds.
Brewster's Angle and Reflection Polarisation
Brewster's angle (θ_B) is the angle of incidence at which light reflected from a polished
gem surface becomes completely plane-polarised. At this angle, the reflected and refracted
rays are perpendicular to each other.
Formula: tan(θ_B) = n (for air/gem interface, n₁ = 1, n₂ = n of gem)
Derivation: At Brewster's angle, the reflected and refracted beams are at 90°. By Snell's
law: n₁ sin(θ_B) = n₂ sin(90° − θ_B) = n₂ cos(θ_B), giving tan(θ_B) = n₂/n₁ = n.
Worked example – diamond:
n(diamond) = 2.417; θ_B = arctan(2.417) = approximately 67.5°.
At this angle, light reflected from the diamond surface vibrates only in the plane
perpendicular to the plane of incidence – fully polarised.
A polaroid filter held at the appropriate angle above a gem surface eliminates surface
glare at Brewster's angle, improving transparency during examination. The immersion method
in the polariscope similarly exploits this suppression of surface reflections.
Source: Read, P.G. "An Experimental Brewster-Angle Refractometer." J. Gemmology, 1979.
DOI: 10.15506/jog.1979.16.8.537 [VERIFIED]; Read, P. "Further development of the
Brewster-angle refractometer." J. Gemmology, 1988. DOI: 10.15506/jog.1988.21.1.36 [VERIFIED]
Brewster angle reference values
The values below are calculated from the formula θ_B = arctan(n) using the known RI
of each gem material. Brewster's angle increases with RI – higher-RI stones require a
steeper angle of incidence to achieve full polarisation of the reflected beam.
| Gem | RI (n, mean or single value) | Brewster angle (arctan n) |
|---|---|---|
| Fluorite | 1.434 | ~55.1° |
| Quartz | 1.548 (mean) | ~57.1° |
| Sapphire/corundum | 1.770 (mean) | ~60.6° |
| Zircon (high) | ~1.92 (mean) | ~62.4° |
| CZ (cubic zirconia) | ~2.17 | ~65.2° |
| Diamond | 2.417 | ~67.5° |
Brewster-angle refractometer
Peter Read developed an experimental Brewster-angle refractometer (1979, 1988) that
extends RI measurement beyond the standard contact-liquid refractometer limit of ~1.79
by detecting the angle at which reflected light becomes fully polarised. This allows
measurement of high-RI stones such as diamond (2.417), CZ (2.17), and high zircon
(up to 1.99).
Dispersion – Named Values (B–G Interval)
Dispersion in gemmology is measured as the difference in refractive index between the
Fraunhofer B line (686.7 nm, red) and the G line (430.8 nm, violet):
Dispersion = n(G) − n(B)
A high dispersion means white light is strongly separated into spectral colours ("fire")
when internally reflected and refracted. Fire is visible to the eye only in stones with
dispersion > ~0.025 under ideal conditions.
High dispersion correlates with: (a) high mean RI, (b) proximity of strong absorption
bands to the visible range (steepening the dispersion curve).
Source: Read, Gemmology 3rd ed. DOI: 10.4324/9780080507224 [VERIFIED];
Nassau, Physics and Chemistry of Color, 2001, pp. 52–56 [VERIFIED]
| Material | Dispersion (B–G) | Fire quality / notes |
|---|---|---|
| Rutile (TiO₂ synthetic) | 0.280 | Extreme; rarely faceted due to very high birefringence |
| Strontium titanate (SrTiO₃) | 0.190 | Synthetic; formerly 'fabulite'; highest of gem simulants |
| Synthetic moissanite (SiC) | 0.104 | Higher than diamond; noticeable fire in faceted stones |
| Cubic zirconia (ZrO₂) | 0.060 | Higher than diamond; conspicuous fire – diagnostic vs. diamond (Read 3rd ed. confirmed) |
| Sphene / titanite | 0.051 | Very high; 'adamantine to sub-adamantine' fire; visible coloured flashes |
| Demantoid garnet | 0.057 | Highest of natural garnets; visible fire even through green bodycolour |
| GGG (gadolinium gallium garnet) | 0.045 | Synthetic diamond simulant |
| Diamond | 0.044 | Benchmark for comparison; high fire by natural gem standards |
| Zircon | 0.039 | High; noticeable fire; high birefringence also contributes to scintillation |
| YAG (yttrium aluminium garnet) | 0.028 | Synthetic; moderate fire |
| Almandine garnet | 0.027 | Moderate; noticeable in bright lighting |
| Spinel | 0.020 | Moderate |
| Corundum (ruby, sapphire) | 0.018 | Low; fire rarely visible in coloured stones |
| Glass (typical) | 0.010–0.020 | Varies widely with composition and heavy-metal content |
| Quartz | 0.013 | Low |
| Fluorite | 0.007 | Very low; 'dead' appearance despite sometimes vivid colour |