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The crystals develop an interlocking texture with some of the trace minerals becoming completely surrounded by later forming crystals.Volcanic rocks, because they are able to cool and crystalize rapidly, have a very fine-grained texture; the individual mineral grains are too small to see easily with the naked eye.Note that the halo has the highest intensity of discoloration near the source, gradually fading with distance in the host mineral to a "fuzzy" edge.
Within these rocks are biotite (an iron-bearing form of mica) and fluorite crystals which bear a relatively uncommon class of tiny, concentric discoloration "haloes" (figure 2).
Granites, for example, have more than 10% quartz and abundant potassium feldspar.
Other plutonic rocks have less quartz and potassium, and different ratios of calcium and sodium feldspar minerals.
In metamorphic rocks, new minerals form that are more stable at higher temperatures and pressures.
Sometimes the minerals segregate into distinct bands.