Gemstones & Diamonds

The world of precious stones.

The main gem families, their characteristics, treatments and known deposits.

All Ruby Sapphire Emerald Spinel Tourmaline Garnets Tsavorite Peridot Opal Aquamarine Diamond

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Corundum

Ruby

Hardness: 9
RI: 1.762 – 1.770
SG: ≈ 4.00

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Corundum

Sapphire

Hardness: 9
RI: 1.762 – 1.770
SG: ≈ 4.00

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Beryl

Emerald

Hardness: 7.5 – 8
RI: 1.565 – 1.602
SG: 2.67 – 2.78

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Spinel

Hardness: 8
RI: ≈ 1.718
SG: 3.58 – 3.61

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Tourmaline group

Tourmaline

Hardness: 7 – 7.5
RI: 1.62 – 1.64
SG: ≈ 3.06

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Garnet group

Garnets

Hardness: 6.5 – 7.5
RI: 1.71 – 1.89
SG: 3.5 – 4.3

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Grossular garnet

Tsavorite

Hardness: 7 – 7.5
RI: ≈ 1.74
SG: 3.57 – 3.65

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Olivine (forsterite)

Peridot

Hardness: 6.5 – 7
RI: 1.65 – 1.69
SG: ≈ 3.34

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Hydrated silica

Opal

Hardness: 5.5 – 6.5
RI: 1.37 – 1.47
SG: ≈ 2.15

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Beryl

Aquamarine

Hardness: 7.5 – 8
RI: 1.57 – 1.58
SG: 2.68 – 2.74

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Carbon

Diamond

Hardness: 10
RI: 2.417
SG: ≈ 3.52

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Gemmological reference

Synoptic table

Usual reference values, for guidance only. A specific stone may differ.

Gem	Family	Hardness	Refractive index	SG	Common treatments
Ruby	Corundum	9	1.762 – 1.770	≈ 4.00	Heating is very common, sometimes leaving flux residues in fissures.
Sapphire	Corundum	9	1.762 – 1.770	≈ 4.00	Heating is very common.
Emerald	Beryl	7.5 – 8	1.565 – 1.602	2.67 – 2.78	Fissure filling with oil or resin is very widespread.
Spinel	Spinel	8	≈ 1.718	3.58 – 3.61	Rarely treated.
Tourmaline	Tourmaline group	7 – 7.5	1.62 – 1.64	≈ 3.06	Heating, irradiation, sometimes fissure filling.
Garnets	Garnet group	6.5 – 7.5	1.71 – 1.89	3.5 – 4.3	Generally untreated.
Tsavorite	Grossular garnet	7 – 7.5	≈ 1.74	3.57 – 3.65	Generally untreated.
Peridot	Olivine (forsterite)	6.5 – 7	1.65 – 1.69	≈ 3.34	Generally untreated.
Opal	Hydrated silica	5.5 – 6.5	1.37 – 1.47	≈ 2.15	Polymer impregnation, "smoke" or "sugar" treatments to darken the body colour, doublets and triplets (assembled stones).
Aquamarine	Beryl	7.5 – 8	1.57 – 1.58	2.68 – 2.74	Heating is common to remove the green or yellow component.
Diamond	Carbon	10	2.417	≈ 3.52	HPHT treatment, irradiation and annealing, laser drilling, fracture filling, coatings.

About origins. The origins listed are known deposits for each species. They never designate the origin of a particular stone, which can only be established on sufficient evidence, as a rule an independent laboratory report. Images marked “Illustration” do not show stones offered by GemAria.

Illustrative image

Ruby

Corundum

Ruby

The red variety of corundum, coloured by chromium. Its value rests first on colour, then on clarity, cutting and weight. Fine unheated rubies above a few carats are rare.

Composition: Al₂O₃ (Cr)
Hardness (Mohs): 9
Refractive index: 1.762 – 1.770
Specific gravity: ≈ 4.00

Colours and varieties: From pinkish red to deep, slightly purplish red. Trade names such as "pigeon's blood" have no single scientific definition.
Distinctive features: Red fluorescence under UV is common in chromium-rich, iron-poor rubies. Typical inclusions: rutile needles ("silk"), crystals, fingerprints.
Treatments: Heating is very common, sometimes leaving flux residues in fissures. Lead-glass filling (must always be identified). Diffusion is less common. A laboratory report is recommended for valuable stones.
Possible origins: Myanmar (Mogok), Mozambique, Madagascar, Tanzania, Thailand and Cambodia, Sri Lanka, Afghanistan, Kenya, Vietnam.

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Sapphire

Corundum

Sapphire

Every corundum that is not red is a sapphire. Blue, caused by iron and titanium, remains the reference, but sapphire also occurs in pink, yellow, green, violet, colourless and colour-change varieties.

Composition: Al₂O₃ (Fe, Ti)
Hardness (Mohs): 9
Refractive index: 1.762 – 1.770
Specific gravity: ≈ 4.00

Colours and varieties: Blue, padparadscha (pinkish orange), pink, yellow, green, violet, colourless, colour-change and star sapphires.
Distinctive features: Colour zoning is common and best seen in immersion. Inclusions and absorption spectra help separate magmatic and metamorphic deposits.
Treatments: Heating is very common. Beryllium or titanium diffusion, fissure filling. Some treatments can only be detected by a laboratory.
Possible origins: Kashmir, Sri Lanka, Madagascar, Myanmar, Australia, Thailand, Montana (USA), Nigeria, Tanzania.

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Emerald

Beryl

Emerald

The green variety of beryl, coloured by chromium and vanadium. Inclusions are part of its identity: a perfectly clean emerald should raise questions.

Composition: Be₃Al₂Si₆O₁₈ (Cr, V)
Hardness (Mohs): 7.5 – 8
Refractive index: 1.565 – 1.602
Specific gravity: 2.67 – 2.78

Colours and varieties: From slightly yellowish green to bluish green. The boundary with iron-coloured "green beryl" is interpreted differently by different laboratories.
Distinctive features: Two- and three-phase inclusions are characteristic of some deposits. Sensitive to knocks, ultrasonic cleaning and heat.
Treatments: Fissure filling with oil or resin is very widespread. Its degree (minor, moderate, significant) strongly affects value.
Possible origins: Colombia, Zambia, Brazil, Afghanistan (Panjshir), Ethiopia, Madagascar, Zimbabwe, Russia.

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Spinel

Long mistaken for ruby, spinel is now sought after in its own right. It is usually offered untreated, which makes it a favourite among connoisseurs.

Composition: MgAl₂O₄
Hardness (Mohs): 8
Refractive index: ≈ 1.718
Specific gravity: 3.58 – 3.61

Colours and varieties: Red, vivid pink, orange, violet, lavender, grey, blue (including the very rare cobalt blue).
Distinctive features: Singly refractive, which separates it from ruby with a polariscope or refractometer. Octahedral inclusions are common.
Treatments: Rarely treated. Heating is occasionally applied. Synthetic spinel is common and must be ruled out.
Possible origins: Myanmar, Tanzania (Mahenge), Sri Lanka, Vietnam, Tajikistan, Madagascar.

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Illustrative image

Tourmaline

Tourmaline group

Tourmaline

A group of chemically complex borosilicates, tourmaline offers the widest palette in the gem world, often within a single stone.

Composition: Elbaite, dravite, liddicoatite…
Hardness (Mohs): 7 – 7.5
Refractive index: 1.62 – 1.64
Specific gravity: ≈ 3.06

Colours and varieties: Rubellite (pink to red), indicolite (blue), verdelite (green), bicolour and "watermelon" tourmalines, copper-bearing Paraíba-type tourmalines.
Distinctive features: Strong birefringence and distinct pleochroism: the orientation of the cut greatly affects the perceived colour.
Treatments: Heating, irradiation, sometimes fissure filling. Many tourmalines remain untreated.
Possible origins: Brazil, Nigeria, Mozambique, Afghanistan, Madagascar, Namibia, USA.

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Garnets

Garnet group

Garnets

Garnet is not one stone but a family of related minerals, ranging from very accessible gems to collectors' rarities.

Composition: Pyrope, almandine, spessartine, grossular, andradite
Hardness (Mohs): 6.5 – 7.5
Refractive index: 1.71 – 1.89
Specific gravity: 3.5 – 4.3

Colours and varieties: Rhodolite (purplish pink), spessartine ("mandarin" orange), demantoid (vivid green), grossulars, colour-change garnets.
Distinctive features: Singly refractive. The "horsetail" inclusions of demantoid are famous.
Treatments: Generally untreated.
Possible origins: Many deposits: Tanzania, Kenya, Madagascar, Mozambique, Namibia, Sri Lanka, India, Russia.

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Tsavorite

Grossular garnet

Tsavorite

Green grossular coloured by vanadium and chromium, discovered in East Africa in the late 1960s. Bright lustre and saturated colour, generally untreated.

Composition: Ca₃Al₂(SiO₄)₃ (V, Cr)
Hardness (Mohs): 7 – 7.5
Refractive index: ≈ 1.74
Specific gravity: 3.57 – 3.65

Colours and varieties: From light to intense green. Fine-coloured stones above a few carats are rare.
Distinctive features: Singly refractive, with a higher refractive index than emerald, from which it is easily distinguished.
Treatments: Generally untreated.
Possible origins: Kenya, Tanzania, Madagascar.

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Peridot

Olivine (forsterite)

Peridot

The gem variety of olivine, a slightly yellowish green caused by iron. It forms in the Earth's mantle and is also found in some meteorites.

Composition: (Mg,Fe)₂SiO₄
Hardness (Mohs): 6.5 – 7
Refractive index: 1.65 – 1.69
Specific gravity: ≈ 3.34

Colours and varieties: Yellowish green to olive green.
Distinctive features: High birefringence: facet edge doubling visible with a loupe. Typical "lily pad" inclusions.
Treatments: Generally untreated.
Possible origins: Pakistan, Myanmar, USA (Arizona), China, Vietnam, Egypt (Zabargad island), Tanzania.

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Opal

Hydrated silica

Opal

Amorphous silica containing water. Precious opal owes its play of colour to an ordered stacking of silica spheres that diffracts light.

Composition: SiO₂·nH₂O
Hardness (Mohs): 5.5 – 6.5
Refractive index: 1.37 – 1.47
Specific gravity: ≈ 2.15

Colours and varieties: Black opal, white opal, boulder opal (on its host rock), crystal opal, fire opal.
Distinctive features: Fragile, sensitive to knocks and temperature changes. Some Ethiopian opals are hydrophane and change appearance when wet.
Treatments: Polymer impregnation, "smoke" or "sugar" treatments to darken the body colour, doublets and triplets (assembled stones).
Possible origins: Australia (black opal, boulder opal in Queensland), Ethiopia, Mexico, Brazil.

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Aquamarine

Beryl

Aquamarine

The blue variety of beryl, coloured by iron. Often very clean, it allows generous cuts and large stones.

Composition: Be₃Al₂Si₆O₁₈ (Fe)
Hardness (Mohs): 7.5 – 8
Refractive index: 1.57 – 1.58
Specific gravity: 2.68 – 2.74

Colours and varieties: From very light to deep blue, sometimes with a green tint.
Distinctive features: Characteristic parallel growth tubes ("rain"). Blue and colourless pleochroism.
Treatments: Heating is common to remove the green or yellow component.
Possible origins: Brazil, Madagascar, Nigeria, Mozambique, Pakistan, Afghanistan.

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Diamond

Carbon

Diamond

Pure carbon crystallised under very high pressure. Diamonds are assessed on the "4Cs": colour, clarity, cut and carat weight. Separating natural from laboratory-grown diamonds is now essential.

Composition: C
Hardness (Mohs): 10
Refractive index: 2.417
Specific gravity: ≈ 3.52

Colours and varieties: Colourless to light yellow (D to Z scale), and fancy colours: yellow, brown, pink, blue, green.
Distinctive features: Synthetic diamonds (HPHT and CVD processes) are chemically identical to natural diamond: detecting them requires specialised instruments.
Treatments: HPHT treatment, irradiation and annealing, laser drilling, fracture filling, coatings.
Possible origins: Botswana, Russia, Canada, South Africa, Namibia, Angola, Democratic Republic of the Congo, Australia.

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Real stones photographed by GemAria. Only the background is retouched: the colour, size and clarity of the stones are not altered.

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Rubellite, round cut

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Parcel of emeralds, emerald cut

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