text: The extent to which a solid is crystalline (crystallinity) has important effects on its physical properties. Sulfur, while usually polycrystalline, may also occur in other allotropic forms with completely different properties. Although crystallites are referred to as grains, powder grains are different, as they can be composed of smaller polycrystalline grains themselves. Generally, polycrystals cannot be superheated; they will melt promptly once they are brought to a high enough temperature. This is because grain boundaries are amorphous, and serve as nucleation points for the liquid phase. By contrast, if no solid nucleus is present as a liquid cools, it tends to become supercooled. Since this is undesirable for mechanical materials, alloy designers often take steps against it (by grain refinement).
  Material fractures can be either intergranular or a transgranular fracture. There is an ambiguity with powder grains: a powder grain can be made of several crystallites. Thus, the (powder) "grain size" found by laser granulometry can be different from the "grain size" (rather, crystallite size) found by X-ray diffraction (e.g. Scherrer method), by optical microscopy under polarised light, or by scanning electron microscopy (backscattered electrons).
  If the individual crystallites are oriented completely at random, a large enough volume of polycrystalline material will be approximately isotropic. This property helps the simplifying assumptions of continuum mechanics to apply to real-world solids. However, most manufactured materials have some alignment to their crystallites, resulting in texture that must be taken into account for accurate predictions of their behavior and characteristics. When the crystallites are mostly ordered with a random spread of orientations, one has a mosaic crystal. Abnormal grain growth, where a small number of crystallites are significantly larger than the mean crystallite size, is commonly observed in diverse polycrystalline materials, and results in mechanical and optical properties that diverge from similar materials having a monodisperse crystallite size distribution with a similar mean crystallite size.
  Coarse grained rocks are formed very slowly, while fine grained rocks are formed quickly, on geological time scales. If a rock forms very quickly, such as from the solidification of lava ejected from a volcano, there may be no crystals at all. This is how obsidian forms.