Last updated on January 3, 2024
Garnet, the official birthstone for January, is generally thought of as red but can be found in a wide variety of colours. This beautiful semi-precious gem is one of the most ancient stones known to man, with a history tracing back more than 5000 years and discovered amongst Ancient Egypt artefacts.
Garnet Origin and Formation
The name garnet comes from the Latin “granatus”, from “granum”, meaning grain or seed, possibly a reference to the vivid red seed covers of the pomegranate, which are similar in shape, size and colour to some garnet crystals. It refers to a large group of silicate minerals that share a similar crystal structure and physical properties but vary in chemical composition.
With different varieties and chemical formulas, garnets can be formed in many ways. However, in general, garnet crystals form when sedimentary rocks are subjected to immense pressure and temperature. Such conditions melt the rock, and as it cools, it crystallizes in cracks and gaps in metamorphic rock surroundings. Once silicate material mixes with other chemical impurities, it creates various garnet colours.
The mineral garnet is found all over the world. It is quite common in its base form and is usually used industrially, for example, as an abrasive material for sandpaper.
Gem-quality garnet is not that easily mined. Different species and types are found in various locations around the world, such as Russia, Namibia, Kenya, Madagascar, Tanzania and Sri Lanka. However, the most significant sources as a whole are the US, Australia, India and China.
There are many varieties of garnets, but not all of them are used for jewellery production. Gem-quality garnets are broken into six main types:
- almandine – the most common type of garnet that is usually dark red to reddish-brown;
- pyrope – the most well-known variety that comes in deep red colour and can rival rubies;
- spessartine – at its best, this type of garnet has a bright orange colour and unique brilliance that makes it a highly sought-after gemstone;
- grossular – in its purest form, this type is colourless; however, impurities can turn it into many beautiful colours and shades, including the rare and very valuable green tsavorite;
- andradite – the most sparkly variety that comes in many colours, commonly in green and black. However, the term andradite is not often used to describe gems. This type is usually broken down into further groups, including highly valued demantoid garnets.
- uvarovite – a unique garnet type that appears only in green colour, usually in small crystals.
These six varieties are formally recognized names for the garnet, but they are further divided into groups depending on their trade names, colours and sources. Some garnets can also be a blend of two or more species. For example, rhodolite garnet is a blend of pyrope and almandine. Malaya or malaia garnet, also known as imperial garnet, is a blend of pyrope and spessartite.
There are also star garnets that have brownish-red or reddish-black colouring and display 4-rayed or even 6-rayed stars. This phenomenon is caused by rutile needle inclusions that align in a specific way to reflect light in a star-shaped pattern.
In general, colour variations in gemstones are caused by impurities within the host crystals. With garnets, it is the same story; however, in some cases, it is hard to tell which mineral causes which colour. It is also worth mentioning that some gemstones, including some types of garnets, get their colour from their basic chemical formula and not from an added impurity.
Red and brown garnets are coloured by the presence of iron during their formation process. Orange and pink garnets get their colour from manganese, yellow from calcium and green from aluminium, chromium or vanadium. Depending on the combination of mineral impurities, garnets get unique colours and shades. For example, an iron and chromium mix produces a purple garnet.
As in the case of all gemstones, rare-coloured garnets are the most expensive. Some of the rarest colour garnets are the rich orange-red spessartine, also known as mandarin garnet, and the trio of rich greens, tsavorite, demantoid and uvarovite. However, the most highly valued type is the colour-change garnet from Madagascar and Tanzania, which appears blue in daylight and red or purple under artificial light. Plain blue garnets do not exist.
As a general rule, garnets with fewer incisions are more valuable. A typical garnet should be eye-clean; blemishes and inclusions are not that tolerated.
However, the story changes a little when it comes to unique inclusions, as they can increase the value of a stone. For example, Russian demantoid garnets can have “horsetail” inclusions, fine golden hair-like flaws within the gem. Such rare inclusions are regarded by gem collectors, as they serve as a reliable indication of their Russian origin.
Another example is the rare and expensive uvarovite garnet. It is an opaque stone, meaning clarity does not come into it. The third premium-priced garnet variety is tsavorite. While clean tsavorites are more valuable, it is expected that a large gem may have some flaws.
Garnets, like many other gemstones, can be cut into various shapes and styles to enhance their beauty and brilliance. The choice of cut can vary based on personal preference, the type of jewellery setting, and the desired style or purpose for the finished piece.
The most common cuts for garnets are the brilliant and step cuts, such as round, oval, cushion, emerald, pear and marquise. However, garnets can also be fashioned into cabochons. Such gems feature a smooth, rounded and polished surface, enhancing the stone’s colour and sometimes displaying a phenomenon like an asterism (a star-like effect) in certain garnet types.
Each cut not only affects the appearance of the garnet but also influences how light interacts with the stone, affecting its brilliance, colour display, and overall aesthetic.
Garnet Carat Weight
The carat weight of a garnet, like other gemstones, refers to its weight and not its size. Carat is a standard unit of measurement specifically used to weigh gemstones, with 1 carat equivalent to 200 milligrams.
Garnets vary in density, meaning two garnets of the same carat weight might have different sizes depending on their specific gravity. Some garnets, like demantoid or tsavorite, tend to have higher densities, making them smaller in size compared to other garnet varieties with the same carat weight.
When it comes to garnets, larger stones are rarer and, therefore, more valuable, especially if they possess good colour, clarity, and cut. However, smaller garnets can also be valuable if they exhibit exceptional qualities or come from rare varieties.
|Almandine - Fe3Al2Si3O12
Pyrope - Mg3Al2Si3O12
Spessartine - Mn3Al2Si3O12
Grossular - Ca3Al2Si3O12
Andradite - Ca3Fe2Si3O12
Uvarovite - Ca3Cr2Si3O12
|Typically red, but can be orange, green, yellow, purple, black, or brown
|6.5 - 7.5 on the Mohs scale
|1.72 - 1.94
|3.5 - 4.3
|Transparent to translucent
|Vitreous to resinous
Featured image: Mikhail Zlatar / Sutterstock