Different mineral ratios determine the kind of igneous rock that forms. There are three ways to compare igneous rock types, color, density and crystal size. Color is known is Felsic or light in color and Mafic or dark in color. Density is determined by comparing Al or aluminum based rocks to Fe or Iron based rocks. Crystal size depends on where the rock formed.
Extrusive rocks cool above ground and cool in a shorter time than Intrusive rocks below ground.
Therefore, Extrusive rocks have smaller crystal sizes or grain sizes than Intrusively formed rocks. The hardest part is using the reference tables. Porforitic igneous rocks are formed when solid chunks of rock fall into magma and become part of a new rock. Using the lab stations around the room identify some of the characteristics for each of the igneous rocks on display. A long time ago I was thrown out of the ground and had a lot of Quartz and Plagioclase Feldspar in me.