The SkyJems Gemstone Encyclopedia

Sapphire

The term sapphire used alone refers to the blue variety, coloured by intervalence charge transfer between iron and titanium. The term fancy sapphire encompasses all other colours, including yellow, green, and purple. Historically, the most revered blue sapphires originated from the marble-hosted deposits of Kashmir, noted for their velvety, cornflower blue, and from the alluvial gravels of Sri Lanka (Ceylon). Significant deposits also exist in Madagascar and Montana (Yogo Gulch). Standard heat treatment is a traditional and widely accepted manufacturing process used to dissolve rutile silk and intensify colour; unheated material is graded and valued separately.

Padparadscha Sapphire

The most elusive of the fancy sapphires is the padparadscha. Named after the Sinhalese word for lotus flower, this variety exhibits a delicate, balanced blend of pink and orange. Originally discovered in Sri Lanka, strict laboratory grading standards require the stone to display this specific pastel colour blend without any dark brownish overtones. Its rarity is reflected in its price.

Star Sapphire & Star Ruby

These stones exhibit asterism, an optical phenomenon. When corundum forms with dense, intersecting networks of microscopic, needle-like rutile inclusions (known as silk), lapidaries cut the stone into a smooth cabochon. When a single, direct light source strikes the dome, the light reflects off the rutile needles to produce a six-rayed star that glides across the surface.

Ruby

Ruby is the red variety of corundum, drawing its colour exclusively from trace amounts of chromium. The most historically important rubies come from the Mogok Stone Tract in Myanmar (Burma), where marble-hosted geology produces stones very poor in iron, which allows for a highly fluorescent, intensely saturated “pigeon’s blood” red. Since the 2010s, large amphibolite-hosted deposits discovered at Montepuez in Mozambique have reshaped the supply side of the market, providing significant new volumes of fine red material.

Emerald (Colombian, Zambian, Muzo, Chivor)

Emerald is the green variety of beryl, coloured by chromium and, in some material, vanadium. The legendary Colombian mines (Muzo, Chivor, La Pita) produce emeralds in unique sedimentary black-shale environments, giving a pure, vivid green. Zambian emeralds, mined primarily at Kagem, are schist-hosted and often yield stones with fewer inclusions and a slightly bluish-green hue. Because of their turbulent geological formation, almost all emeralds feature jardins — internal fractures and inclusions. Consequently, the long-standing manufacturing practice of filling these fissures with colourless oil, traditionally cedarwood, to improve apparent clarity, remains a standard and accepted trade practice today, and should always be disclosed at the point of sale.

Aquamarine

Named after seawater, aquamarine is the blue to slightly greenish-blue variety of beryl, coloured by trace amounts of iron. Unlike emerald, aquamarine crystals often grow to large sizes in granitic pegmatites, which allows cutters to fashion clean gems frequently exceeding ten to twenty carats. Its clarity and size potential make aquamarine a classic choice for cocktail rings, favoured by Art Deco designers and by contemporary designers for the same reasons.

Morganite & Heliodor (Golden Beryl)

Morganite is the pink to peach variety of beryl, coloured by manganese. It was named after the American financier J. P. Morgan and has become a widely used centre stone for rose-gold engagement rings. Heliodor, or golden beryl, draws its sun-kissed yellow hues from iron. Both offer good brilliance, solid durability, and an accessible entry point into the precious beryl family.

Tsavorite & Demantoid (The Green Garnets)

Tsavorite is a vivid green variety of grossular garnet. It was discovered in the 1960s near Tsavo National Park in Tanzania and Kenya. Its bright green colour rivals emerald, and it has superior durability and is almost never treated. Demantoid is the chromium-green variety of andradite garnet. It is known for its diamond-like dispersion (fire) and is especially prized when it contains radiating, fibrous horsetail inclusions of chrysotile — a feature associated most famously with material from the Ural Mountains in Russia.

Spessartite, Rhodolite, Pyrope & Hessonite

• Spessartine (spessartite): an idiochromatic garnet coloured by manganese, known for vivid, fiery “Fanta” or mandarin-orange hues.
• Rhodolite: a hybrid of almandine and pyrope, prized for its rich purplish-red to raspberry tones.
• Pyrope: the classic, deep, dark blood-red garnet historically favoured in Bohemian (Czech) and Victorian jewellery.
• Hessonite: a variety of grossular garnet, known for warm, cinnamon-orange to brownish-red colour.

Tourmaline (General & Bi-colour)

Standard tourmalines exhibit a broad range of colours, including vibrant greens, pinks, and yellows. Because the chemical environment often changes while the crystal is growing, tourmalines frequently display distinct colour zones within a single crystal. This produces bi-colour gems such as the well-known watermelon tourmaline, which features a pink core surrounded by a green rind.

Rubellite & Indicolite

Rubellite is the pink to purplish-red variety of tourmaline, coloured by manganese. The finest rubellites, historically sourced from the Ouro Fino mine in Brazil, display a pure cherry red that can approach the appearance of ruby. Indicolite is the blue to greenish-blue variety, coloured by iron. Both are strongly pleochroic, which means lapidaries must orient the rough carefully to present the best colour through the table facet.

Paraíba Tourmaline

Discovered in the late 1980s in the Brazilian state of Paraíba, this is the most valuable tourmaline variety on the market. Its vivid neon or electric blue-green colour is caused by the rare presence of copper (Cu²⁺) in the crystal lattice. Because natural copper-bearing crystals often carry purplish overtones, they are routinely heat-treated to resolve to the pure neon-blue hues; treatment status is disclosed at the point of sale.

Tanzanite

Tanzanite is the blue to violet gem variety of the mineral zoisite. It is a single-source gemstone, found only in the Merelani Hills of northern Tanzania, where it was discovered in 1967. Tanzanite is notable for its strong trichroism — showing distinct blue, violet, and burgundy-red colours when viewed from different crystallographic directions. Because the rough crystals emerge from the earth with a dominant brownish-green axis, nearly all tanzanite is gently heated to around 600°C to eliminate the brown component and lock in the blue-violet colour. This treatment is standard and is disclosed at the point of sale.

Spinel

Spinel (magnesium aluminium oxide) is the great historical imposter of the gem world. Because it occurs in the same geological deposits as ruby and sapphire — for example the marble-hosted deposits of Myanmar and Tajikistan — ancient civilisations did not distinguish between them. The famous Black Prince’s Ruby in the British Imperial State Crown is, in fact, a large red spinel. Today, spinel is recognised on its own merits: it occurs in vivid reds, pinks, and blues, has excellent durability (Mohs 8), and, unlike corundum, is almost never heat-treated.

Amethyst, Citrine, Ametrine & Prasiolite

• Amethyst: the purple variety of quartz, historically reserved for royalty and clergy. Its colour is caused by iron impurities and natural irradiation.
• Citrine: the golden-yellow to orange variety of quartz.
• Ametrine: a naturally bi-coloured quartz displaying zones of both purple amethyst and yellow citrine. Its sole commercial source is the Anahí mine in Bolivia, where specific hydrothermal conditions produced the two colours in tandem.
• Prasiolite: a pale leek-green variety of quartz, typically produced by the careful heating of specific types of amethyst; treatment is disclosed at the point of sale.

Agate, Carnelian & Onyx (Chalcedony)

These are varieties of chalcedony — cryptocrystalline quartz composed of submicroscopic fibres. Agate is characterised by concentric, curved colour banding. Carnelian is a translucent, brownish-red to bright orange chalcedony coloured by iron oxide. Onyx features flat, parallel colour banding (typically black and white), which has made it the historical material of choice for carving cameos and intaglios.

Opal (Australian, Boulder, Black & Fire)

Unlike most gemstones, opal is amorphous — it lacks a rigid crystal lattice and consists of hydrated silica spheres. Precious opal exhibits play-of-colour, an optical phenomenon in which light diffracts through a tightly packed grid of microscopic silica spheres to produce flashes of spectral colour. Black opal (primarily from Lightning Ridge, Australia) is the most valuable variety: its dark body tone provides a stark contrast that makes the colour flashes stand out. Fire opal, mined primarily in Mexico, is a distinct type; it is prized for its transparent-to-translucent orange-red body colour and often lacks play-of-colour entirely, which allows it to be faceted rather than cut as a cabochon.

Ammolite

Although ammolite is often grouped visually with opal because of its similar iridescent play-of-colour, it is an organic gemstone. It is formed from the fossilised, crushed shells of ancient ammonites (extinct marine molluscs) found primarily in the Bearpaw Formation of Alberta, Canada. The sheet-like iridescence is caused by light interference rebounding through microscopic, stacked layers of aragonite platelets.

Chrysoberyl & Cat’s Eye Chrysoberyl (Cymophane)

Faceted transparent chrysoberyl typically exhibits bright, highly refractive yellow-to-green hues caused by trace iron. The species is best known, however, for its phenomenal variety, cymophane, or cat’s eye. When chrysoberyl crystallises with thousands of microscopic, parallel rutile tubes, lapidaries cut the stone as a cabochon (a smooth dome). Light striking the parallel tubes creates a sharp, concentrated band of light across the dome — an optical effect known as chatoyancy. True cat’s-eye chrysoberyl often displays a characteristic “milk and honey” effect, in which one side of the stone appears milky and the other golden brown when illuminated from across the stone.

Alexandrite

Alexandrite is the colour-change variety of chrysoberyl, originally discovered in the Ural Mountains of Russia in the 1830s and named after Tsar Alexander II. The optical phenomenon is produced by the substitution of chromium ions in the crystal lattice: the crystal absorbs light in the yellow region of the spectrum and transmits both red and green. In daylight (rich in blue-green light) the stone appears green; under incandescent light (rich in red wavelengths) it shifts to purplish red. With the Russian deposits largely worked out, the finest modern alexandrites come from Brazil and Madagascar.

Zircon (including Blue Zircon)

Zircon (zirconium silicate, ZrSiO₄) is an important natural gemstone that is often confused by consumers with the unrelated synthetic diamond simulant cubic zirconia. Natural zircon crystals are in fact among the oldest known terrestrial materials, with some samples dated to about 4.4 billion years. Zircon possesses a high refractive index and strong dispersion (fire) that approaches diamond, and it is strongly birefringent, which often produces visible doubling of the back facets. It occurs naturally in earthy browns and greens (known as “high” and “low” zircons depending on the degree of natural radioactive metamictisation). The electric-blue variety — typically produced by heating Cambodian rough — is the most commercially popular.

Topaz

Topaz (aluminium silicate fluoride hydroxide) is a strong jewellery stone with a Mohs hardness of 8. Historically the name topaz was applied loosely to any yellow gem; true mineralogical topaz is comparatively uncommon. The most valuable variety is Imperial topaz, which exhibits a peachy-orange to reddish-pink colour and is mined predominantly at Ouro Preto in Brazil. Conversely, the majority of blue topaz on the global market begins as colourless natural material that is subjected to electron-beam or gamma irradiation followed by gentle heating to produce stable, saturated blue hues (Swiss Blue, London Blue, and others). This treatment is standard and is disclosed at the point of sale.

Peridot

Peridot is the gem-quality variety of the mineral olivine. It is an unusual idiochromatic gem, which means its colour comes from the basic chemistry of the mineral itself (iron) rather than from a trace impurity. Peridot therefore exists in only one colour: yellowish-green to olive green. It is typically formed in the intense heat of the earth’s mantle and brought to the surface by volcanic basalts, and has also been recovered from pallasite meteorites. Its high birefringence produces a distinct doubling of internal inclusions, particularly the “lily pad” inclusions characteristic of Pakistani material.

Kunzite (Spodumene)

Kunzite is the pink to lilac variety of the mineral spodumene, coloured by trace manganese. It was named after the American gemologist George F. Kunz, who first described it in 1902 from deposits in San Diego County, California. Kunzite is strongly pleochroic (showing different colours from different crystal directions) and has two directions of perfect cleavage. These properties make it technically demanding to cut. Cutters must orient the table facet perpendicular to the c-axis to capture the deepest pink through the face of the stone.

Sphene (Titanite)

Sphene takes its name from the Greek word for wedge, reflecting the shape of its crystals; mineralogists refer to it as titanite. It is valued by collectors for one specific optical property: dispersion. Sphene has a dispersive power (the ability to split white light into flashes of spectral colour) higher than that of diamond. Well cut, a yellow-green sphene will show a strong display of multicoloured fire. With a hardness of only 5 to 5.5, however, sphene is best reserved for pendants, brooches, and occasional wear.

Kyanite, Andalusite & Iolite

These gems are distinct studies in crystallography and optical physics.

Kyanite is an aluminium silicate known for an intense, sapphire-like blue. It is highly anisotropic in hardness: about 4.5 along the length of the crystal, and about 7 across its width.

Andalusite shares the chemical formula of kyanite (they are polymorphs) but crystallises in a different system. It is prized for strong pleochroism, showing a simultaneous interplay of olive green and reddish-brown (sometimes called its earthy colours) within the same faceted stone, depending on the viewing angle.

Iolite, the gem variety of cordierite, is sometimes called the Viking compass or water sapphire. It exhibits strong trichroism; a single iolite crystal can appear violet-blue from one direction, clear as water from another, and yellowish-brown from a third.

Collector Rarities: Apatite, Benitoite, Fluorite, Sphalerite & Rhodochrosite

This category covers the rarer fringes of gemology. Many of these species possess physical characteristics — extreme softness or perfect cleavage — that make them collector pieces, better suited to display or careful occasional wear than to daily use.

Sphalerite (zinc sulphide) has a high refractive index and dispersion approximately three times that of diamond, but a Mohs hardness of only about 3.5. Benitoite is an exceptionally rare barium titanium silicate found almost exclusively in San Benito County, California, with a sapphire-like blue and strong dispersion. Apatite can display neon blue-green colours comparable to Paraíba tourmaline. Fluorite and rhodochrosite offer vivid purples and banded pinks respectively, but require careful lapidary handling.

Pearl

Pearls are unique among gems because they are produced organically by living molluscs. When an irritant enters the shell, the mollusc secretes concentric layers of nacre — a combination of crystalline aragonite and an organic binder called conchiolin — around it. The optical effect known as orient is produced by light refracting through these microscopic overlapping platelets. The modern market is dominated by cultured pearls, in which human intervention initiates the growth process. Notable varieties include the classic Japanese Akoya, South Sea pearls (known for their large size), and the black and grey pearls produced by Pinctada margaritifera in Tahiti and French Polynesia.

Jade (Nephrite and Jadeite)

Historically treated as a single material, modern gemology recognises jade as two distinct, extremely tough aggregate minerals: nephrite (an amphibole silicate) and jadeite (a pyroxene silicate). Jadeite is the rarer and more valuable of the two, particularly the translucent, chromium-rich “Imperial jade” from Myanmar. Because jade is frequently treated, the trade uses a strict nomenclature: A-jade refers to entirely natural, untreated material (save for traditional surface waxing), while B-jade has been bleached with acid to remove brown staining and then impregnated with polymer resins to improve stability and apparent transparency. Careful collectors insist on A-jade and require disclosure of any treatment.

Lapis Lazuli

Lapis lazuli is not a single mineral but a rock: a complex aggregate composed primarily of deep-blue lazurite, often mottled with white calcite and brassy flecks of pyrite. Mined in the Sar-e-Sang deposits of Afghanistan for more than six thousand years, lapis was ground into powder by Renaissance artists to create the pigment ultramarine and was used extensively in the burial objects of Egyptian pharaohs.

The Feldspars: Moonstone, Labradorite & Sunstone

Feldspar is the most abundant mineral group in the Earth’s crust, but specific gem varieties show distinct optical phenomena produced by light scattering between microscopic, alternating layers of different feldspar compositions.

Moonstone displays adularescence, a billowing, ghostly blue or white light that floats across the gem’s surface. Labradorite exhibits labradorescence, a metallic iridescence of blues, greens, and golds. Sunstone is prized for aventurescence, a sparkling, glittery effect caused by flat, highly reflective metallic inclusions (typically copper or hematite) suspended within the crystal.

Turquoise, Malachite & Chrysoprase

These opaque to highly translucent gems are known for their distinctive body colours. Turquoise (a hydrous phosphate of copper and aluminium) and malachite (a copper carbonate) owe their blue and banded green hues to the presence of copper. Chrysoprase, an apple-green variety of chalcedony, derives its colour from trace nickel, which distinguishes it from chromium-coloured green agates.

Diamond

Diamond is the only gemstone composed of a single element: pure carbon. Crystallised under extreme pressure and temperature deep within the Earth’s mantle, it has a Mohs hardness of 10, which makes it the hardest known natural material. In a coloured-gemstone context, high-quality colourless diamonds are principally used as brilliant accent stones — in halos or as side stones — to contrast with and support a central coloured gem. Naturally coloured “fancy” diamonds (canary yellows, pinks, blacks) form their own tier of the market.

Mandarin Garnet

A trade name applied to exceptional, vividly saturated orange spessartite garnets. The term gained prominence in the 1990s following the discovery of neon “Fanta” orange material in the Kunene region of Namibia, and subsequently in Nigeria. Because of its electric colour and the fact that the material is untreated, “Mandarin” garnet commands a premium over other spessartite.

Chrome Diopside

Chrome diopside is a calcium magnesium silicate coloured by chromium. It is mined almost exclusively in the harsh, frozen environment of Siberia, Russia, and offers a rich forest-green colour that can serve as an affordable alternative to tsavorite garnet or emerald. Its relative softness (Mohs 5.5 to 6) and perfect cleavage make it prone to scratching and require careful setting, which limits its use in daily-wear rings.