The stones, read closely.

Formation

Commonly associated with marbles formed where limestones were metamorphosed, and with some basalt-related and metamorphic settings. Chromium must be present while iron is limited for the finest red.

Characteristics

Hardness 9. Strong red fluorescence in many chromium-rich stones. Fine rutile needles ('silk') are a characteristic inclusion; heat treatment is common and must be identified.

Major producing regions

Formation

Forms in metamorphic rocks and is also carried to the surface by alkali basalts. Many important sources are secondary gem gravels where durable corundum has concentrated.

Characteristics

Hardness 9, high durability and brilliance. Colour zoning, silk and fingerprints are typical internal features; heat treatment is widespread.

Major producing regions

Formation

Requires beryllium and chromium or vanadium to meet — elements rarely found together. Colombian emeralds form from hydrothermal fluids in black shales; many other sources relate to schists at contacts with pegmatites.

Characteristics

Hardness 7.5–8 but often fractured. Inclusions ('jardin') are expected; clarity enhancement with oils or resins is common and must be disclosed.

Major producing regions

Formation

Characteristically forms in granitic pegmatites, crystallising from boron-rich late-stage melts and fluids. Colour zoning records the changing chemistry of that growth.

Characteristics

Hardness 7–7.5, strong pleochroism and elongated, striated prismatic crystals. Copper-bearing 'Paraíba-type' tourmaline shows vivid neon blues and greens.

Major producing regions

Formation

Precious opal forms where silica-rich water fills voids and fractures in weathered sedimentary rocks, depositing ordered stacks of microscopic silica spheres.

Characteristics

Hardness 5.5–6.5 and contains water, so it can be sensitive to heat and sudden changes. The size and order of the silica spheres produce its play-of-colour.

Major producing regions

Formation

Commonly crystallises inside gas cavities in volcanic basalts, where silica-rich fluids deposit quartz crystals over time. Also found in veins and pegmatites.

Characteristics

Hardness 7. Colour is often zoned and can fade with prolonged heat or strong light; heating can turn amethyst yellow to orange.

Major producing regions

Formation

Grows in granitic pegmatites, often as large, clean crystals. The blue comes from iron; many stones are gently heated to remove greenish tones.

Characteristics

Hardness 7.5–8, typically eye-clean, with hexagonal prismatic crystals. Distinct pleochroism in blue and near-colourless tones.

Major producing regions

Formation

Forms in pegmatites, rhyolites and high-temperature veins where fluorine-rich fluids are present. Brazil's Ouro Preto district is known for 'imperial' golden to pinkish-orange topaz.

Characteristics

Hardness 8 but with perfect cleavage, so it can split with a sharp blow. Most blue topaz on the market is produced by irradiation and heating of colourless material.

Major producing regions

Formation

Widespread in metamorphic rocks such as schists and gneisses, and in some igneous rocks. Individual species record the pressure and temperature conditions of their formation.

Characteristics

Hardness 6.5–7.5, singly refractive, with high brilliance. Species include pyrope, almandine, spessartine, grossular (tsavorite) and andradite (demantoid).

Major producing regions