Gemstone reference table

Key properties of the most common gemstones: hardness, specific gravity, refractive index and the treatments you are likely to meet.

Gem	Mohs	SG	RI	Common treatments	AGTA	Ultrasonic	Steam	Heat and chemicals	Plating with the stone setRhodium or gold: dip / pen
DiamondDiamond	10	3.523.50–3.53	2.417	HPHT annealing, irradiation, fracture filling, laser drilling, coating	HP, R, F, L, C	Usually safe*	Usually safe*	Very stable. *Not for fracture-filled or coated diamonds. Cleaners can loosen stones in worn settings.	Dip: Usually safe* Pen: Usually safe Takes the baths well. *Not fracture-filled or coated diamonds: pen-plate around them.
RubyCorundum	9	4.003.97–4.05	1.762–1.770	Heat (most stones), lead-glass filling, beryllium diffusion	H, F, U	Usually safe*	Usually safe*	Stable to heat, light and chemicals. Boric acid etches the surface. Filled or dyed stones: damp cloth only; mild acids damage fillers.	Dip: Usually safe* Pen: Usually safe Takes the baths well. *Lead-glass-filled rubies: the acid dip and bath attack the glass; pen-plate and keep the solution off the stone.
SapphireCorundum	9	4.003.95–4.03	1.762–1.770	Heat (most stones), beryllium or titanium diffusion	H, U	Usually safe*	Usually safe*	Stable to heat, light and chemicals. Boric acid etches the surface. Filled or dyed stones: damp cloth only.	Dip: Usually safe* Pen: Usually safe Takes the baths well. *Not filled, dyed or coated stones: pen-plate and keep the solution off the stone.
EmeraldBeryl	7.5–8	2.722.67–2.78	1.577–1.583	Oil or resin in fractures (very common)	O	Not recommended	Not recommended	Heat extends fractures; ultrasonic and steam can weaken stones and sweat out oil or resin fillers. Warm soapy water only.	Dip: Not recommended Pen: Use care Ultrasonic, steam and the hot cleaner can draw out oil or resin fillers. Pen-plate with the solution kept off the stone, or plate before setting.
AquamarineBeryl	7.5–8	2.722.68–2.74	1.577–1.583	Heat (to remove green)	H	Usually safe*	Usually safe*	Avoid heat. Colour stable to light. Attacked by hydrofluoric acid.	Dip: Usually safe* Pen: Usually safe Takes the baths well. *Not filled, dyed or coated stones: pen-plate and keep the solution off the stone.
MorganiteBeryl	7.5–8	2.802.66–2.87	1.583–1.590	Heat, sometimes irradiation	H, R	Usually safe*	Usually safe*	Avoid heat. Colour stable to light. Attacked by hydrofluoric acid.	Dip: Usually safe* Pen: Usually safe Takes the baths well. *Not filled, dyed or coated stones: pen-plate and keep the solution off the stone.
AlexandriteChrysoberyl	8.5	3.733.70–3.78	1.746–1.755	Usually none (synthetics are common)	N	Usually safe	Usually safe	Stable to heat, light and common chemicals.	Dip: Usually safe Pen: Usually safe Takes the baths well.
SpinelSpinel	8	3.603.54–3.63	1.718	Usually none; occasionally heat	N, H	Usually safe*	Usually safe*	High heat can fade some colours; stable to light and chemicals.	Dip: Usually safe* Pen: Usually safe Takes the baths well. *Not filled, dyed or coated stones: pen-plate and keep the solution off the stone.
TopazTopaz	8	3.533.49–3.57	1.619–1.627	Blue: irradiation and heat; pink: heat; coatings	R, H, C	Not recommended	Not recommended	High heat or sudden temperature change can break it; some colours fade in heat or sunlight; coatings wear.	Dip: Use care Pen: Usually safe Skip ultrasonic and steam and avoid sudden temperature changes between warm baths and cold rinses. Coated topaz: pen only, off the stone.
TourmalineTourmaline	7–7.5	3.062.82–3.90	1.624–1.644	Heat, irradiation (pink/red), sometimes filling	H, R, F	Not recommended	Not recommended	Heat can alter colour; sudden temperature change can fracture it.	Dip: Use care Pen: Usually safe Skip ultrasonic and steam; sudden temperature change can fracture it. Pen plating is the safer choice.
Almandine garnetGarnet	7–7.5	4.053.93–4.30	1.79	Usually none	N	Usually safe*	Not recommended	Stable to light and chemicals; attacked by hydrofluoric acid.	Dip: Usually safe* Pen: Usually safe Takes the baths well; skip the steam cleaner.
Pyrope / rhodolite garnetGarnet	7–7.5	3.783.62–3.87	1.746	Usually none	N	Usually safe*	Not recommended	Stable to light and chemicals; attacked by hydrofluoric acid.	Dip: Usually safe* Pen: Usually safe Takes the baths well; skip the steam cleaner.
Tsavorite garnetGarnet	7–7.5	3.613.56–3.73	1.740	Usually none	N	Usually safe*	Not recommended	Stable to light and chemicals; attacked by hydrofluoric acid.	Dip: Use care Pen: Usually safe Heat-sensitive (GIA): skip steam and keep baths warm, not hot. Pen plating is the safer choice.
Spessartine garnetGarnet	7–7.5	4.154.12–4.18	1.80	Usually none	N	Usually safe*	Not recommended	Stable to light and chemicals; attacked by hydrofluoric acid.	Dip: Usually safe* Pen: Usually safe Takes the baths well; skip the steam cleaner.
Demantoid garnetGarnet	6.5–7	3.843.70–4.10	1.888	Sometimes heat	N, H	Usually safe*	Not recommended	Stable to light and chemicals; attacked by hydrofluoric acid.	Dip: Use care Pen: Usually safe Often included and treated: skip steam, avoid temperature shocks. Pen plating is the safer choice.
PeridotOlivine	6.5–7	3.343.28–3.48	1.654–1.690	Usually none	N	Not recommended	Not recommended	Rapid or uneven heat can fracture it. Attacked by sulfuric and hydrochloric acid, and slowly by acidic perspiration.	Dip: Not recommended Pen: Use care Attacked by sulfuric acid, which is in the activation dip and rhodium bath. Pen-plate with the solution kept off the stone.
TanzaniteZoisite	6–7	3.353.35	1.691–1.700	Heat (almost always)	H	Never	Never	No high temperatures or sudden temperature changes. Attacked by hydrochloric and hydrofluoric acid.	Dip: Not recommended Pen: Use care Never ultrasonic or steam; sudden temperature changes can crack it. Pen-plate with the solution kept off the stone.
ZirconZircon	6–7.5	4.703.93–4.73	1.925–1.984	Heat (blue and colourless)	H	Not recommended	Not recommended	Heat can alter colour; some heat-treated stones revert after long exposure to bright light.	Dip: Use care Pen: Usually safe Skip ultrasonic and steam; heat can alter colour. Pen plating is the safer choice.
AmethystQuartz	7	2.662.65	1.544–1.553	Sometimes heat	N, H	Usually safe*	Not recommended	No heat: sudden temperature changes can fracture it. Colour can fade in intense light. Attacked by hydrofluoric acid and alkaline solutions.	Dip: Use care Pen: Usually safe Skip steam; sudden temperature change can fracture it and hot alkaline solutions attack quartz. Keep the electroclean short.
CitrineQuartz	7	2.662.65	1.544–1.553	Usually heated amethyst	H	Usually safe*	Not recommended	No heat: sudden temperature changes can fracture it. Colour can fade in intense light. Attacked by hydrofluoric acid and alkaline solutions.	Dip: Use care Pen: Usually safe Skip steam; sudden temperature change can fracture it and hot alkaline solutions attack quartz. Keep the electroclean short.
OpalOpal	5–6.5	2.151.25–2.23	1.45	Impregnation (oil, wax, plastic), smoke or sugar treatment, doublets and triplets	I, D, ASBL	Never	Never	High heat or sudden temperature change can crack it; intense light can cause crazing. Attacked by hydrofluoric acid and alkaline solutions.	Dip: Never Pen: Not recommended Contains water: heat, temperature changes and alkaline solutions crack or dull it. Plate before setting, or remove the opal.
Jadeite jadeJadeite	6.5–7	3.343.30–3.38	1.66	Bleaching + polymer impregnation (“B jade”), dyeing (“C jade”), wax	B, I, D, W	Usually safe*	Usually safe*	Torch heat harms jade; affected by warm acids. *Safe only for untreated jade.	Dip: Use care Pen: Usually safe* Untreated jade only: warm acids affect it. *Polymer-impregnated (B) jade: plate before setting.
Nephrite jadeNephrite	6–6.5	2.952.90–3.03	1.61	Wax, sometimes dye	W, D	Usually safe*	Usually safe*	Torch heat harms jade; affected by warm acids. *Safe only for untreated jade.	Dip: Use care Pen: Usually safe* Warm acids affect it; untreated only. *Treated nephrite: plate before setting.
TurquoiseTurquoise	5–6	2.762.31–2.84	1.61	Stabilisation (polymer/wax impregnation), dyeing	I, D, W	Never	Never	High heat discolours it; chemicals, cosmetics, skin oils and perspiration can discolour it. Dissolves slowly in hydrochloric acid.	Dip: Never Pen: Not recommended Porous: absorbs and is discoloured by the solutions and heat. Plate before setting.
Lapis lazuliLapis lazuli	5–6	2.752.50–3.00	1.50	Dyeing, wax or oil	D, W	Not recommended	Not recommended	Dye can rub off with acetone or alcohol; wax sealers break down with heat or solvents.	Dip: Not recommended Pen: Use care Dye and wax can come out in the hot cleaner. Pen-plate with the solution kept well away, or plate before setting.
Pearl (cultured)Pearl	2.5–3	2.702.60–2.85	1.53–1.69	Bleaching (routine), dyeing, irradiation, coating	B, D, R, C	Never	Never	Heat burns or cracks pearls. Damaged by all acids, perfume, hair spray, cosmetics and perspiration.	Dip: Never Pen: Not recommended Acids dissolve nacre and heat cracks it. Plate before the pearl is set or glued on.
MoonstoneFeldspar	6–6.5	2.562.56–2.59	1.518–1.526	Usually none	N	Never	Never	High heat or sudden temperature change can break it; attacked by hydrofluoric acid.	Dip: Not recommended Pen: Use care Never ultrasonic or steam; temperature changes can break it. Pen-plate with the solution kept off the stone.
KunziteSpodumene	6.5–7	3.183.15–3.21	1.660–1.676	Irradiation and/or heat	R, H	Not recommended	Not recommended	Heat or sudden temperature change can break it; colour fades with heat or intense light.	Dip: Use care Pen: Usually safe Skip ultrasonic and steam; heat can fade it and temperature changes can break it. Pen plating is the safer choice.
IoliteCordierite	7–7.5	2.612.58–2.66	1.542–1.551	Usually none	N	Not recommended	Not recommended	High heat or sudden temperature change can break it.	Dip: Use care Pen: Usually safe Skip ultrasonic and steam; temperature changes can break it. Pen plating is the safer choice.
CoralCoral	3–4	2.652.60–2.70	1.49–1.66	Bleaching, dyeing, polymer impregnation	B, D, I	Never	Never	Porous and soft: keep away from heat, acids, cosmetics and perfume. Rinse briefly; do not soak.	Dip: Never Pen: Not recommended Porous and dissolved by acids. Plate before setting.
AmberAmber	2–2.5	1.081.05–1.09	1.54	Heat/clarifying, reconstruction (pressed amber)	H	Not recommended	Not recommended	Burns at low temperatures; darkens with long light exposure; damaged by chemicals. Warm soapy water only.	Dip: Never Pen: Not recommended Softens and is damaged by heat and chemicals. Plate before setting.

Bench care from GIA's care and cleaning guides. * Not for fracture-filled, cavity-filled, dyed or heavily included stones. Warm soapy water is safe for every gem in this table.

Plating: JewelLoop bench guidance. A dip takes the whole piece through ultrasonic and steam cleaning, a hot alkaline electrocleaner (about 49–65 °C), a sulfuric acid activation dip, a warm acidic rhodium bath (about 32–46 °C) and cold rinses (Stuller Bench Jeweler; MJSA/Ganoksin); gold plating uses the same steps and its own warm bath. Pen (brush) plating treats only the metal it touches, so a sensitive stone is safe as long as the solution never reaches it. “Not recommended” for pen means: plate before the stone is set. Each rating follows the stone's sensitivity in GIA's care guides. When in doubt, ask your plater or remove the stone.

AGTA treatment codes

N: Not enhanced, or no known enhancement
B: Bleaching
C: Coating
D: Dyeing
F: Filling (glass, plastic or similar in fractures or cavities)
H: Heating
HP: Heat and pressure (HPHT)
I: Impregnation (usually plastic or polymer in porous stones)
L: Lasering (laser drilling, mostly diamonds)
O: Oiling or resin infusion
R: Irradiation
U: Diffusion
W: Waxing or oiling of opaque stones
ASBL: Assembled (doublets, triplets)

Estimates for guidance; verify before production.

Interactive Gem property lookup Enter the readings from your refractometer, hydrostatic balance and polariscope. The page shows which of 47 gems and common look-alikes in the reference table have those properties, and where your readings fall on a map of them all. Open

Enter your refractometer readings and specific gravity, or tap an example reading to see how the search works.

Interactive Hardness and toughness A hard gem resists scratches; a tough gem resists breaking. They are not the same thing, and the difference decides which stones belong in a daily-wear ring and which in earrings. Every rating here is GIA's own wording. Open

Choose two stones and press Strike it to compare them.

Understand the result

Using the table

Hardness (Mohs) shows how well a stone resists scratching, which matters for rings and everyday wear. Specific gravity (SG) and refractive index (RI) are the first tests gemmologists use to identify a stone.

Treatments and disclosure

The treatment column lists what is common in the market, with the American Gem Trade Association (AGTA) codes. It is a summary: every stone must be disclosed individually, and only a laboratory can confirm what was done to a specific gem.

Care at the bench

The ultrasonic, steam and heat columns follow GIA's care guides. Treated stones are more fragile than untreated ones: when in doubt, clean with warm soapy water and a soft brush, and tell your setter and polisher what the stone is.

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