TRAVERTINE
Travertine is a sedimentary rock.
Travertine is a natural chemical
precipitate of carbonate minerals;
typically aragonite, but often
recrystallized to or primarily
calcite; which is deposited from the
water of mineral springs (especially
hot springs) or streams saturated
with calcium carbonate. When pure,
travertine is white, but often is
brown to yellow due to impurities.
When carbon dioxide-rich water
percolates through rocks in
limestone areas, the water dissolves
the limestone and becomes saturated
with it. When the water resurfaces
later, the sudden drop in pressure
and the change in temperature cause
the water to release the carbon
dioxide gas, much like fizzy drinks.
The calcium carbonate then
recrystallizes, often over minute
underwater plants. The resulting
rock is typically quite porous with
numerous cavities. When
exceptionally porous it is known as
calcarious tufa.MARBLE
Marble is a metamorphic rock
resulting from the metamorphism of
limestone, composed mostly of
calcite (a crystalline form of
calcium carbonate, CaCO3). It is
extensively used for sculpture, as a
building material, and in many other
applications. The word 'marble' is
colloquially used to refer to many
other stones that are capable of
taking a high polish.
Pure white marble is the result of
metamorphism of very pure limestones.
The characteristic swirls and veins
of many colored marble varieties are
usually due to various mineral
impurities such as clay, silt, sand,
iron oxides, or chert which were
originally present as grains or
layers in the limestone. Green
coloration is often due to
serpentine resulting from originally
high magnesium limestone or
dolostone with silica impurities.
These various impurities have been
mobilized and recrystallized by the
intense pressure and heat of the
metamorphism.
ONYX
Onyx is a banded variety of
chalcedony, a cryptocrystalline form
of quartz. The colors of its bands
range from white to almost every
color (not including shades such as
purple, blue, or black). Commonly,
specimens of onyx available contain
colors of white, tan, and brown.
Sardonyx is a variant in which the
colored bands are sard (shades of
red) rather than black.
It is usually cut as a cabochon, or
into beads, and is also used for
intaglios and cameos, where the
bands make the image contrast with
the ground. Some onyx is natural but
much is produced by the staining of
agate.
LIMESTONE
Limestone is a sedimentary rock
composed largely of the mineral
calcite (calcium carbonate: CaCO3).
Limestone often contains variable
amounts of silica in the form of
chert or flint, as well as varying
amounts of clay, silt and sand as
disseminations, nodules, or layers
within the rock. Pure limestone is
white or almost white. Because of
impurities, such as clay, sand,
organic remains, iron oxide and
other materials, many limestones
exhibit different colors, especially
on weathered surfaces. Limestone may
be crystalline, clastic, granular,
or massive, depending on the method
of formation. Crystals of calcite,
quartz, dolomite or barite may line
small cavities in the rock. Folk and
Dunham classifications are used to
describe limestones more precisely.
SLATE
Slate is a fine-grained,
homogeneous, metamorphic rock
derived from an original shale-type
sedimentary rock composed of clay or
volcanic ash through low grade
regional metamorphism. The result is
a foliated rock in which the
foliation may not correspond to the
original sedimentary layering.
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