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This
Quartz crystal's dark smoky color is due to a disruption of its crystal lattice
(framework) due to irradiation.
A doubly terminated Quartz crystal tinted with a small amount of iron to give it
a purple color. This variety of quartz is Amethyst. This crystal is
from Vera Cruz, Mexico.
Two
more specimens of amethyst from Vera Cruz, Mexico
Amethyst Quartz crystal cluster from Date Creek, AZ
Aqua Quartz
CRYPTOCRYSTALLINE QUARTZ - This is the name given to aggregates of extremely small quartz crystals that commonly form in a sedimentary environment, usually by silica rich solutions. There are a tremendous number of forms, colors and textures for cryptocrystalline (hidden crystals) quartz. Varieties include a multitude of various forms of agate, chert, and flint. Here are some samples:
Chalcedony is a cryptocrystalline variety
of Quartz.
Bulls eye Agate
These are slabs (thin slices) of
Agate from Brazil. Like the Bullseye Agate above they show a concentric
pattern. These types of agate are formed by the filling of a cavity.
Quartz rich solutions fill the void and the microscopic crystals of quartz (the
agate) grow as a layer around the void. Sometimes the solutions,
temperatures or pressures change slightly and create an area of larger crystal
growth. These larger crystals can be seen in the above examples. The
coloration commonly found in agate is due to iron oxides - providing earth
tones.
Here is another slab from Brazil
showing very nice zonation of the crystal growth.
Here are some additional pictures of zoned agate from Brazil. The "earth-tones" are produced by the presence of iron as an impurity.
Agate slab with close-up on
right.
