A porphyroblast is a texture in many metamorphic rocks that have certain minerals that are larger than the rest of the rock matrix in grain size. The large grains are known as porphyroblasts and the rock has a porphyroblastic texture. Many porphyroblasts form in pelitic rocks. Some of these minerals are garnet, staurolite, cordierite, kyanite, sillimanite, and andalusite. In general the higher the metamorphic grade the larger the porphyroblasts can become. Different minerals form at different temperatures and pressures so rocks such as quartzite and marble that only contain one mineral do not form prophyroblasts.
Garnet porphyroblasts in a pelitic rock.