Crystallites are small

Crystallite size is usually measured from X-ray diffraction patterns and grain size by other experimental techniques like transmission electron microscopy. Solid objects that are large enough to see and handle are rarely composed of a single crystal, except for a few cases (gems, silicon single crystals for the electronics industry, certain types of fiber, single crystals of a nickel-based superalloy for turbojet engines, and some ice crystals which can exceed 0.5 meters in diameter[1]). Most materials are polycrystalline; they are made of a large number of single crystals — crystallites — held together by thin layers of amorphous solid. The crystallite size can vary from a few nanometers to several millimeters.
If the individual crystallites are oriented randomly (that is, if they lack texture), 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, which must be taken into account for accurate predictions of their behavior and characteristics.
Material fractures can be intergranular fracture 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" (or, 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).

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