Natural Science in Archaeology

46 3 Exploitation of Mineral and Rock Raw Materials
Problems with nomenclature – establishing which ancient rock and mineral
names can be correlated with modern usage – extend beyond puzzling about Pliny’s
terminology. Feldman (1973) has provided some correlations and ethnohistoric
data on rock and mineral names used in studies of ancient Mesoamerica. Some
names such as basalt, syenite, porphyry, ophiolite, and alabaster have their origins
in ancient Egypt. Basalt appears to have the oldest roots, which can be traced back
to a Pharaonic rock name in use as early as 2000 BCE. For a discussion of these
name origins, see Harrell (1995).
3.2 Rock Classification and Properties
A rock is an aggregate of one or more specific minerals that occurs commonly
enough to be given a name (e.g., granite, sandstone). Rocks are often referred to as
“stone”, a word that has many meanings. Within the archaeomineralogical context
the usage should be restricted to “building stone” and “gemstone”, but this book
uses the word stone in the sense of each original author when referring to ancient
or other texts that use the term “stone” whether or not the rock or mineral type is
known.
The three major rock types are igneous (from the Latin ignarious “belonging to
fire”), those that formed from molten magma; sedimentary, those formed from the
consolidation of deposited clastic particles or formed by precipitation from solution;
and metamorphic (from the Greek for change of form), those formed by major
alteration of preexisting rocks by the action of high temperature and pressure.
Naming of rock types is based on component minerals (reflecting the chemical
composition) and texture. In the field, it may be difficult to positively identify the
component minerals, especially in fine-grained rocks, so names are generalized.
When rock specimens are available for laboratory analyses, petrography can be
used for positive identification and more specific names can be assigned.
The relative proportions of the constituent minerals in a rock determine its two
most readily perceived properties – color and density. In igneous rocks the lightcolored
(high silica) minerals are of relatively low density (mostly 2.5–2.7), and the
dark-colored (low silica) minerals are significantly denser (mostly 3.0–3.6). Rocks
have no specific hardness, except for monomineralic rocks. However, the hardness
of any rock is influenced by the minerals that make it up. For example, rocks such
as granite, composed of quartz (hardness 7) and feldspar (hardness 6), are obviously
harder than marble, composed of calcite (hardness 3).
3.3 Igneous Rocks
Igneous rocks, which result from the cooling and crystallization of molten magma,
are divided into two major textural categories. Those that crystallize within the