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Metamorphic rocks represent alterations of precursor sedimentary, igneous, or other metamorphic rocks.
Through the cycles of burial, folding, faulting, and subduction of crustal plates, rocks get pushed and dragged down to depths where - under heat and pressure - changes take place.
Discoloration haloes in younger rocks tend to be smaller and less intense than in older rocks, indicating that the zone of crystal damage increases with time.
From these observations early attempts were made to use the dimensions of haloes as an age dating technique.
When these minerals occur as inclusions in certain other minerals, most notably the mica family, they are often seen to develop discoloration, or "pleochroic" haloes.
The haloes are caused by radiation damage to the host mineral's crystalline structure.
Other plutonic rocks have less quartz and potassium, and different ratios of calcium and sodium feldspar minerals.True granites are relative latecomers on the geologic scene as they required a number of recycles of crustal material to differentiate and concentrate potassium.