Beryl, Menzies, Wester Australia, Australia
Beryl is a typical mineral of pegmatites, although some hydrothermal deposits contain gem-quality beryl crystals. Emerald, the green variety, is coloured by small amounts of chromium. Beryl is the main ore mineral of the light, strong metal beryllium, used in the aerospace industry.
Source: Museum Victoria
Spessartine specimen from Broken Hill, NSW, Australia
Garnets are a family of aluminosilicate minerals that share the same crystal structure. They form beautiful crystals that can sometimes be used as gems. The wide variety of different chemical compositions results in a range of colours. Garnet gems include grosular, andradite, almandine and spessartine.
Source: Museum Victoria
Uncut Sapphires, Mornington Peninsula, Victoria, Museum Victoria
Corundum is one of the hardest minerals, and its crystals come in many colours. Red (ruby) and blue (sapphire) varieties are common gemstones, and are coloured by trace amounts of chromium and iron/titanium respectively. Fine-grained, poor-quality corundum (emery) is used as an abrasive.
Image: Frank Coffa
Source: Museum Victoria
Zircon, Strangways Range, Northern Territory, Australia
Small zircon crystals occur in many rocks, but crystals large enough to make gems are uncommon. Red to brown crystals are eroded from basaltic volcanic rocks and concentrated into sands and gravels. Large brown zircon crystals from the Harts Range in central Australia formed in unusual carbonate-rich igneous rocks.
Source: Museum Victoria
Amblygonite, Ceara, Brazil
Almost any attractive transparent or translucent crystal can be cut into a gem. But some minerals are not durable enough be used in jewellery. Many other minerals are not well known or are very rare, so gems made from them are mainly of interest to collectors.
Source: Museum Victoria
Topaz, Juab County, Thomas range, Utah, USA
Topaz is an aluminium fluorosilicate that is often found as colourless to blue crystals in pegmatites and in hydrothermal tin deposits. Topaz breaks easily along planes of weakness perpendicular to the length of the crystal. This property can make it difficult to facet as a gem.
Image: unnknown
Source: Museum Victoria
Quartz, variety citrine
Quartz is the most abundant mineral on the Earth’s surface. It comes in a wide variety of forms and colours. Most is milky white, but pale yellow, brown, purple and rare pink varieties are caused by chemical impurities or radiation. Quartz enclosing needles of brown rutile make interesting gemstones.
Image: Benjamin Healley
Source: Museum Victoria
Quartz, variety ametrine, Santa Cruz, Bolivia
Image: Benjamin Healley
Source: Museum Victoria
Opal, Quilpie, Queensland, Australia.
Opal consists of tiny spheres of silica containing small amounts of water. In precious opal these spheres have a regular size and form a three-dimensional lattice that diffracts light, creating amazing plays of colours. But in common or ‘potch’ opal the spheres vary in size and cannot bend light in the same way.
Source: Museum Victoria
Forsterite crystal from the Suppat region, Pakistan
As a gemstone, forsterite is called peridot and it is the birthstone for August.
Image: Benjamin Healley
Source: Museum Victoria
Gemstones have been prized for thousands of years for their colour, clarity, rarity, shape and durability. They are made from minerals that form in many different geological situations. Gemstones are cut and polished to enhance their natural beauty and remove imperfections. The smooth facets created in this way help to transmit and reflect light.
Some gemstones are more valuable than others. Depth and evenness of colour are important, and intensely coloured stones are especially sought after. A gemstone’s colour depends mainly on its chemical composition. In some gemstones the main element causes the colour; for example, turquoise — copper phosphate — is coloured blue by the copper. In other gemstones, very small amounts of an element cause the colour.
Museum Victoria’s collection of faceted gemstones and polished ornamental rocks has been acquired since the 1860s. New discoveries mean there are many more minerals used as gemstones now than in the past. Significant contributions to the gemstone collection have been made recently by Peter Hoppen, Murray Thompson, Ron Perrin and Grant Hamid.