Colors of Gemstones, Precious Stones, Precious Metals

For thousands of years, humans have valued Earth's natural resources for ornaments and jewelry. Gemstones are rocks or minerals valued for their beauty and rarity.  Most gems are found deep in Earth's crust.   Gems are usually crystals.  Naturally occurring gems formed in conditions of heat, pressure and millions of years.

Synthetic gems can be grown in the laboratory and are identical in structure to natural gems.  They can be difficult for even experienced geologists to tell apart.  Diamonds, rubies, sapphires, emeralds and many others are available as synthetics.

Opals are fascinating gemstones that flicker and change like moods. They seem to capture fire and oceans, lightning and rainbows. Almost 95% of fine opals are mined from Australian deserts. Opal can be opaque or translucent. The translucent opals show the play of colour and are highly sought after.

Opals are actually made from colourless silca, the mineral in quartz. The difference is that the silica is hydrated - they contain around five percent of water trapped with pockets of silica spheres. The hydrated silica acts like a prism that diffracts (splits) white light into rainbow colours.

Small spheres of silica produce blue and green colours. Larger spheres produce rarer red colours and orange colours. Opals with red will also produce all the other colours in the rainbow which increases its value.

Crystals used to make gemstones are formed deep in Earth's crust. Larger crystals form when molten rock cooled slowly. Small crystal form when molten rock cools quicker. Obsidian (volcanic glass) is not a crystal - it cooled to quickly for crystals to form.

Contaminants produce colour. Pure quartz crystals are made from colourless silica, used to make glass. Amethyst (purple) is quartz contaminated with iron or manganese. Quartz crystals are long prisms with three-fold symmetry.

Diamond is composed of evenly spaced carbon atoms. Carbon atoms in a different arrangement form slippery grey-black graphite.

Under extreme heat and pressure within Earth's crust, carbon crystallized out as diamonds. Diamond crystals shapes include octahedral, cubic and dodecahedron.

Most of the world's diamonds are from Africa. Diamond is so hard, that it is used for industrial drills.

Rubies, sapphires and emeralds are typically found as tubular crystals in a hexagonal shape with blunt ends.

Peridot is a yellow-green gem that was prized by ancient Egyptians and was often confused with emeralds in royal jewels. Peridot is a transparent, gem variety of olivine. The colour is from iron.

Transparent crystals with vivid colour are most sought after as gemstones. Transparent crystals are cut with flat faces (facets) that display optimal clarity and brilliance. Light is reflected (bounced) and refracted (bent) within the crystal and the angles of the facets is critical for maximum brilliance.

Diamonds are usually colourless.  They are exceptional at reflecting, refracting and diffracting light.  Skillfully faceted diamonds are the most sparkly of gems.  All other gemstones are softer than diamond.  

Different contaminants in the same mineral can result in different colours. Chromium in beryl makes emeralds (green). Iron in beryl makes aquamarine (light blue) or heliodor (yellow).

Corundum with chromium produces red and pink rubies. Corundum with titanium or iron produces many shades of blue sapphires (most common) as well as pink, yellow and green sapphires.

Blue, pink and red topaz is rare in nature. Most blue topaz is produced by irradiating and heat treating pale topaz. Other colours of topaz can be made artificially, including green, pink and 'mystic' (iridescent)

The same contaminants in different minerals can result in different colours. Chomium in corundrum produces red rubies. Chromium in beryl produces green emeralds. Chromium in crysoberyl produces a alexandrite - a gem that changes colours according to light source: green in sunlight or in fluorescent light; red in incandescent light from an indoor light bulb.

Opaque precious and semi-precious stones can be tumbled with abrasive for hours to smooth and polish them.

Gemstones for jewelry are typically cut with a domed upper surface and flat base to show off colour and pattern and to hide scratches. Usually, gems are cut in an oval, rather than a circle, which disguises imperfections better.

Opaque stones include:

• jasper (many colours including red, purple, yellow, pink, brown)
• turquoise (light blue)
• azure (deep blue)
• quartz with stripes/bands
• malachite (deep green)
• jade/greenstone (semi-translucent green)
• opal - also translucent (huge array of colours and descriptions)

Greenstone is a type of jade traditionally used by New Zealand Maori for carving. It is a very hard metamorphic rock. Maori call greenstone Pounamu.

Maori carved pendants and other ornaments out of Pounamu and also bone.  Each piece is hand-carved in the shape of a traditional symbol such as a fish hook or unfolded fern frond ('koru').

Maori also used Paua shell (abalone shell) as the eyes of traditional wood carvings.  Iridescent blue-green Paua shell is also used to make jewellery.

Gold, silver and copper have been used for thousands of years for decorative purposes. Copper was also used for tools, weapons and implements. Gold, silver and copper can all be found free in Earth's crust. Ancient people such as the Egyptians smelted copper from and other metals such as lead from minerals. The Egyptians used the silvery looking lead-containing mineral Galena as an eye paint.

Pure gold is yellow in colour. It is too soft to use in jewelry, so is blended with other metals to make alloys which have a slightly different colour. White gold alloys contain gold and nickel or palladium. Rose gold contains gold and copper. Gold is resistant to tarnishing and is rare, therefore is highly valued.

Silver is a metallic whitish-grey colour. The exterior tarnishes to black if exposed to hydrogen sulfide or sulfur (common in volcanic regions). Tarnishing is a chemical reaction.

Copper is a metallic pink-orange-brown colour. It forms a green tarnish. The Statue of Liberty has an external copper layer which has a dull green patina from a chemical reaction.

Smooth, polished metals appear lustrous and shiny because they reflect nearly all light. Their different colours arise because they absorb a small amount of certain wavelengths of light.

Not all ornamental gems are from Earth's crust.

Amber has been used for ornamental purposes for thousands of years. It is fossilized resin of ancient conifers and is a mixture of organic hydrocarbons. Frequently, extinct insects, pollen and other inclusions are trapped inside. With time, the resin has become a polymer, a natural plastic.

Pearls and corals are produced by marine animals and both are used in ornamentation.

Pearls are formed by the secretion of molluscs deposited around a foreign body such as a grain of sand.  Pearls occur naturally in a range of colours including white, black, grey, green, pink, blue, yellow, cream and brown.  The colour of pearls depend on the species of mollusc, salinity of water, dissolved minerals in water, algae and contaminants.

Photographic guide to opals

All about gems