Large crystals ( phenocrysts ) set in a fine-grained groundmass (indicates a two-stage cooling process).
Distinguish between the fresh surface (broken with a hammer) and the weathered surface . Weathering can hide the rock's true color or highlight certain mineral textures. 2. Texture (The Cooling History) Texture is the most critical diagnostic feature: The Field Description of Igneous Rocks
Note patterns like columnar jointing (hexagonal columns) or cooling cracks. Large crystals ( phenocrysts ) set in a