TOUGHNESS
Toughness in a gemstone is a measure of how well it can resist a fracture developing, i.e. how far a crack will propagate. Gemstones that are polycrystalline (made of more than one crystal/grain), such as nephrite ,jadeite, and AGATE (polycrystalline quartz variety), are generally tough and difficult to break. The interlocking crystal grains are oriented in different directions, thereby slowing or stopping the propagation of the fracture.
zircon is brittle, it is easily chipped and the corners break off as it cleaves easily. A great deal of care and attention is therefore required at all stages of fashioning by the lapidary, and later in terms of storage by the retailer and ultimately the owner.
