Andalusite is a gemstone known for its strong pleochroic effect, which allows it to appear as anything from green to red or brown depending on the angle from which it is viewed. While the mineral Andalusite is relatively common, gemstone quality transparent Andalusite is pretty rare. Andalusite is not well known to the general population, and although it is gaining popularity for use in jewelry, for the moment is still considered more of a collectors gemstone rather than jewelry gemstone.
Etymology & Sources
Andalusite is named for Andalusia, the region of Spain where the high clarity gemstone variety of the mineral was discovered. There are many deposits of Andalusite found around the world, but most of this is the mineral form and the vast majority is far from gemstone quality. Transparent, gem appropriate Andalusite is largely sourced from Brazil.
Due to its pleochroism, Andalusite can’t be described as a single color as the color changes depending on the viewing angle, but generally speaking the gemstone ranges from green or yellow to red or brown. Depending on how the gem is cut, Andalusite will often appear as if it is a clear blend of two distinct colors, giving it quite a unique appearance. Andalusite can range from transparent all the way to opaque, but as we mentioned earlier, the transparent variety is much rarer and the translucent or opaque forms are much more abundant. Andalusite luster can range from matte to vitreous, and is also sometimes found rutilated, which is caused by needle like inclusions in the structure of the gemstone.
Gem quality, high clarity Andalusite is quite hard,with a 7.5 on the Mohs scale. Opaque varieties, sometimes known as chiastolite, are considerably softer. Andalusite has moderate density, with specific gravity of around 3.15, and has a refractive index level of approximately 1.63.
Gem quality Andalusite is most typically faceted into standard gemstone cuts, but opaque varieties are sometimes cut in cabochons. Correct faceting is incredibly important with Andalusite as a poor cut will diminish the visible pleochroism that makes the gemstone desirable. Andalusite is suitably hard and can be used for any type of jewelry, but is more commonly sold as loose gemstones as it remains more of a collectors stone rather than a gemstone used for jewelry.
Chiastolite – Opaque, softer, and less valuable form of Andalusite
Viridne – Green variety of Andalusite that purportedly gets its color from manganese inclusions.
Andalusite gemstones are typically sold unenhanced and there are no common treatments that are used on it.