Although this word has many different meanings throughout science, in gemmology it is used to describe gemstones whose crystal structures have different hardness in different directions. its hardness varies depending on which axis of the gem you are looking at. In one direction it measures 4.5 – 5 on the Mohs scale and on the other axis it measures 6.5 – 7.
Also known as anisotropism, this effect is one of the best ways for those that evaluate gems to correctly identify Kyanite. Although its polymorphic structure may aid gemmologists in identifying Kyanite, it causes Lapidarists a real headache when cutting the gem. If the orientation is not perfect, then the polymorphic structure causes the gem to literally fall apart! So much so, that until recently the gemstone was only ever cabochon cut. If you do manage to purchase a faceted piece, don’t worry about this, as once cut the gem is durable.
