Fracture is an important parameter when you compare Sillimanite and Onyx Physical Properties. It is necessary to understand the significance of these properties, before you compare Sillimanite Vs Onyx fracture. Whenever a gemstone chip breaks, it leaves a characteristic line along its breakage. Such lines are known as fracture and are used to identify the gemstones in their initial stages of production when they are in the form of rough minerals. Fracture is usually described with the terms “fibrous” and “splintery” to denote a fracture that usually leaves elongated and sharp edges. Fracture observed in Sillimanite is Splintery.
A primary knowledge about Sillimanite vs Onyx luster is useful in apparent identifications of these gemstones. Luster is the measure of light that gets reflected when incident on a finished cut gemstone. There are two major types of lusters: Silky and Adamantine. Since luster varies between two crystals of even the same gemstone, luster is limited to basic identification criteria. Sillimanite exhibits Vitreous, Subadamantine and Silky luster. Onyx, on other hand, exhibits Vitreous luster.