Turquoise is often marked with dark veins
that run through it called "matrix."
In some regions such markings are considered
to be beautiful. This seems to be so
in the U.S. Southwest and the Far East.
In other areas, such as the Middle East,
these markings are thought of as
imperfections and the stones that carry them
are valued less.
Turquoise matrix is essentially the remnants
of the rock that "hosted" the turquoise as
it formed through weathering and oxidation
processes over millions of years.
Turquoise matrix can take on different
colors, depending on the host rock
(sometimes called "the mother rock").
Black matrix tends to be
favored, as it creates a nice contrast.
Matrix of this color is often iron
pyrite (iron sulfide).
Yellow matrix is often
rhyolite, an igneous, volcanic rock.
Since turquoise usually forms in rock
with a volcanic origin, the presence of
rhyolite should not be surprising.
Brown matrix usually
consists of an iron oxide, of which
there are sixteen different types.
The best known of the iron oxides is
The term "spider webbing" is
used to describe turquoise with thin
lines of matrix running throughout them,
much like a spider's web.