In a given igneous rock, the major light colored minerals will be K-feldspar, plagioclase, quartz, and/or muscovite.

• Muscovite is easiest because of its silvery clear color and perfect flaky cleavage. If in doubt, use a needle or knife to see if you can flake the mineral into thin sheets.
• Quartz grains tend to be translucent gray, rounded to irregular blobs. The lack of a cleavage means their exposed surfaces are rounded and bumpy.
• K-feldspar and plagioclase are blocky, not translucent, and show flat cleavage surfaces. They occur in shades of opaque pink, white, gray, or dark gray. Early-formed feldspars tend to be rectangular; late formed feldspars fill in gaps between other minerals and thus are irregular in shape.
• Plagioclase often shows striations whereas K-feldspar does not. If a rock has two feldspars, carefully search for striations to learn which is which. In very dark rocks it is tough to distinguish the boundaries of one black mineral from another. In these rocks, plagioclase crystals frequently create long, thin rectangular crystals that, upon close inspection, show striations on their shiny cleavages.

The major dark colored minerals in a typical igneous rock will be biotite, hornblende, augite, and/or olivine:

• Biotite is easy because it forms perfect shiny black cleavages and can be peeled into flakes. It can be confused with hornblene, but hornblende does not flake.
• Hornblende and augite are often confused. Most reliable is cleavage: hornblende forms ~120°/60° cleavages whereas augite forms ~90° cleavages. In addition, hornblende tends to form very black and shiny cleavages, whereas augite forms duller black and not-very-reflective cleavages. Finally, hornblende tends to form long rectangular crystals whereas augite forms blocky crystals.
• Olivine is a glassy olive green and tends to form rounded, lumpy crystals.