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  • Writer's pictureAntiques in Oxford

All about garnets

Garnets come in many colours and even in colour-changing varieties. They are formed from a group of minerals that have similar crystal structures, often used in jewellery. The first colour that comes to mind is the intense red, typical of almandine and pyrope garnets. However, they also occur in green, yellow, orange, pink, purple and black.


History

Garnets have been around for thousands of years – the Egyptians used them in jewellery. Greeks and Romans called them ‘anthrax’, which means ‘coal’, and ‘carbunculus’ used for red gemstones and still nowadays associated with red garnets.

Throughout the centuries people associated these stones with mystical powers. People wore them as talismans for protection in battle. They were also considered to cure various blood-related illnesses.

The present-day name ‘garnet’ originates from the Latin word ‘granatum’. This means ‘pomegranate’ which is due to the similarity of garnet crystals to pomegranate seeds. In nature, garnet crystals are found in the form of rhombic dodecahedrons*, icositetrahedrons** or a combination of these two.

*Rhombic dodecahedrons means it is a polyhedron with 12 sides

**Icositetrahedrons means it is a polyhedron with 24 sides

Minerals

Garnets are a species of silicate minerals, which are the most common minerals on Earth. They are a compound of silicon and oxygen.

We use six types of these stones in jewellery:

  • almandine

  • pyrope

  • spessartine

  • andradite

  • grossular

  • uvarovite

In nature, garnets are rarely found in their pure form and are often a combination of two or more of these types. These combinations occur during the growth of the minerals, and, in the case of garnets, they form two isomorphous* series. These series, by definition, have the same crystal structure. However, the main compound of silicon+oxygen (SiO4)3 is bound to different atoms. Consequently, there are various colours and slightly different properties, caused by the alternative elements.

*Isomorphous means ‘same form’ – ‘iso’ meaning ‘equal’ and ‘morphous’ meaning ‘form’.


Almandine, pyrope & spessartine garnets

The first isomorphous series we will look at is X3Al2(SiO4)3 – magnesium, iron and manganese are represented by the ‘X’. In this instance, they combine with aluminium. Consequentially, these garnets have a red-purple-orange colour. This combination creates almandine, pyrope and spessartine garnets. These varieties are very popular and commonly found in jewellery. Possibly the most admired of this intermediate-range garnet is ‘rhodolite’, the trade name for a combination between pyrope and almandine which produces an intense purplish-red colour.


Andradite, grossular and uvarovite garnets

The other isomorphous series is Ca3Y2(SiO4)3. Here Calcium is bound to Y which is iron, aluminium, or chromium. This combination produces andradite, grossular and uvarovite variations.

These stones have a wide variety of colours, including yellow, orange, red, pink, green, and black and are mined all over the world. These locations are known for the specific colours that characterise the material mined there.

Demantoid garnets for instance are part of the andradite species. They typically have a bright and intense green mainly due to the chromium content. Demantoid garnets were first discovered in Russia, where the higher-quality gemstones still come from. They are one of the most valuable gemstones on the market.

A more recent discovery is the tsavorite garnet, part of the grossular garnet series. A British geologist discovered tsavorite in Tanzania in 1967. When it entered the market in the 70s, people quickly became fans of this new stone because of its intense emerald-green colour. In addition to this, it also has an increased hardness and a cheaper price tag





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