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Stones are made up of a mosaic of interlocking mineral grains that have definite
chemical compositions. Minerals have been divided into groups. The groups we
are primarily concerned with are the common rock forming mineral groups which can be subdivided into two mineral categories: Silicates and Carbonates.
Silica is the most common chemical component of the earth. Silicates which contain
silica are generally the harder minerals and the most common rock-forming minerals.
These stones can be referred to as Silecous based stones.
Carbonates are the next most important minerals in the rock-forming group. This
mineral group generally contains the softer minerals. These stones can be referred to
as calcareous based stones.
Rocks have been divided into major and minor groups according to the process
by which they are formed and each of these groups are subdivided into further
stone categories. The three major groups from which dimensional stones
being used commercially will be covered.
Igneous : This group has been formed from molten or partly molten material
i.e. from magma. Lava is a form of magma that has cooled and
solidified on the surface of the earth.
Granite is a igneous rock that has cooled and crystallized below
the earth's surfaces, having a fine to coarse-grained mineral crystal
structure. It is a siliceous based stone. Composed primarily of
quartz, feldspar, and other accessory minerals.
Sedimentary : There are four main types of sedimentary rocks.
1. First there are the types that are the result of some preexisting rock's
demise through the weathering process.
2. The second kind is biogenic, stone produced by living organisms such
as marine creatures or vegetation which eventually died and the remains
became part of the sediment on the bottom of ancient sea beds.
3. The third is produced by inorganic chemical material being deposited
on the floor of ancient sea, lake beds and springs through the process
of precipitation or evaporation.
4. The last sedimentary category are produced as a result of volcanic
activity. This stone is composed of volcanic ash with a variety of
secondary older rock materials from the surrounding area, volcanic
and igneous in nature.
Metamorphic : Metamorphic is derived from the Greek meta denoting a change
and morph meaning a change in form. This changing of
one rock type into another new type takes place by an increase in
temperature or pressure or a combination of both. The original
minerals of the stone recrystallize, small crystals merge to form larger
crystals with no changes in the mineral chemistry, fine-grained calcite
in limestone recrystallizes to a coarse-grained calcite crystal structure
in marble; or there may by a transformation into a different set of
metamorphic minerals.
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