Natural Science in Archaeology

4.2 Microcrystalline Quartz 81
after heat treatment. Chalcedony from the Tertiary era Chadron Formation of a fourstate
region centered in northwestern Nebraska, USA, was used from Paleoindian to
Protohistoric times over a wide geographic area to manufacture lithic tools (Hoard
et al. 1992). Variants of the material, based on color and texture of the artifacts, are
known from numerous sites over a wide geographic area.
Agate is a translucent cryptocrystalline variety of quartz frequently mixed with
some opal. It is characterized by variegated colors in bands, cloud-like, or moss-like
forms, in virtually all colors. It is found in vugs or cavities of rocks. As a cryptocrystalline
variety of quartz it breaks with a conchoidal fracture and has been used for
chipped stone tools (Fig. 4.7).
All these materials are chemically precipitated silica. Most cherts are derived
from the silica in limestones as the chert becomes segregated as nodules or discontinuous
beds. Cherts are nearly ubiquitous throughout the world. An X-ray
examination by Heaney and Post (1992) of 150 specimens of fine-grained quartz
varieties from around the world revealed that from 10–80% of the silica in many
samples is moganite, a little-known silica poly-morph. Large amounts of moganite
were prevalent in cherts from arid, alkaline environments. This could be important
in provenance studies.
In North America, the increased interest in technical studies on chert and related
materials has led to the discovery of many prehistoric quarries and prompted
basic geologic studies on the stratigraphic and geographic distribution of cherts
in limestones (see, for example, Butler and May 1984). The most famous prehistoric
chert quarry in eastern North America is the Dover Quarry in Tennessee. The
quarry pits cover about 2 ha. Large chert nodules and tabular pieces were made into
hoes, adzes, and ceremonial objects. Ceremonial swords reached a length of 69 cm
(Butler and May 1984; Gramly 1992a). The Late Plains Archaic Schmitt chert mine
in the Horseshoe Hills of Montana supplied lithic material from 3100 BCE on from
the chert-bearing layers of the Madison Limestone Formation. Many hammerstones
used to break up the bedrock have been found at the mine site(s) (Davis 1982).
The ancient Maya exploited a large chert- and chalcedony-bearing zone in
northern Belize for the production of chipped stone artifacts (Hester and Shafer
1984; Cackler et al. 1999). The chert is found as nodules in limestone. Several lithic
workshops are located in this zone. The occurrence of black chert and chalcedony
artifacts at many archaeological sites in Belize has been of special interest. The
Fig. 4.7 Predynastic agate
blade from Wadi Bezeh,
Egyptian Eastern Desert