One of the hardest things in geologyno, not gneissis the terminology. If you find a big mineral grain, or just a big thing, inside another rock, what do you call it? The experts have words for each possibility: phenocryst, porphyroblast or megaclast. If you've seen these terms in the literature and despair of telling them apart, here's a key to their identities: look at the suffixes. The -crysts, -blasts and -clasts almost always correspond to igneous, metamorphic and sedimentary rocks. Read the gritty details in this article.