Discovering Gemstones - January Edition
Garnet, the birthstone of January, is mined all over the world. But did you know that the term garnet does not only refer to the popular red variety? It encompasses a group of minerals that come in a rich palette of colours, from green to orange and even blue, depending on the trace elements found within its structure!
Garnet has been used as a gemstone for over 5,000 years, making appearances in the jewelry of many Egyptian burials. The red variety was especially popular in Ancient Rome, where signet rings with carved garnets were used to stamp the wax that secured important documents.
But decoration is not this mineral’s only benefit. It is also extremely durable, making it ideal for use as an industrial mineral. In the past 150 years, garnet sand has been used to cut steel and other materials through sandblasting and waterjet cutting.
There are more than twenty different garnet minerals that all have essentially the same crystal structure, but vary in chemical composition. But only five varieties are commercially sold as gems:
|Almandine||Most widely used; only less common, transparent dark red forms are used as gemstones.|
|Pyrope||Most widely used; noted for its transparency and frequent lack of flaws or inclusions. Rhodolite is well-known rose-red to violet variety.|
|Spessartite||Orange to orange-red; recently increased in popularity.|
|Grossular||Most varicoloured form; Tsavorite is an important green variety and Hessonite is a yellow to yellow-green form.|
|Andradite||Most lustrous; has rare green Demantoid variety, the yellow Topazolite variety and the black Melanite variety.|
Garnets generally form as a result of high grade metamorphism deep within the Earth, but can also form close to the surface. Under the right temperature, pressure and composition garnets can crystallize and be found in different rock types such as gneiss and schists. Starting as tiny grains that enlarge slowly over time, they can displace, replace and/or include the surrounding rock materials.
Searching for Diamonds
Garnet is also used as an indicator mineral during mineral exploration for diamonds. Although most garnets are formed within Earth’s crust, some garnets are brought up from the mantle during deep-source volcanic eruptions. When pieces of mantle rock, called xenoliths, make it to the surface they form in a structure known as a “pipe”. These xenolith pipes are the source of most diamonds found at or near Earth’s surface, so if you ever find a garnet in a xenolith, there might be diamonds too!
Check out the new Canadian Diamond exhibit at Dynamic Earth to learn more about indicator minerals.