1. Size: This refers to the individual mineral crystals or fragments that make up the rock. Grains can be very small (microscopic), fine, medium, coarse, or very large (even meters across).

2. Shape: Describes the form of the individual grains. They can be rounded, angular, irregular, elongated, or platy.

3. Texture: This refers to the overall arrangement and relationship of the grains within the rock. Examples include:

* Crystalline: Interlocking crystals, often found in igneous rocks.

* Clastic: Loose fragments cemented together, typical of sedimentary rocks.

* Foliated: Flattened or elongated grains aligned in a preferred direction, common in metamorphic rocks.

Understanding a rock's grain helps geologists identify its formation process and classify its type.