Natural Science in Archaeology

224 10 Abrasives, Salt, Shells, and Miscellaneous Geologic Raw Materials
of most deposits is black and, whatever its composition, it is a heavy, tough rock
with an average hardness of about 7.25. Emery is formed mainly by contact metamorphism
and occurs in irregularly shaped pods in limestone, schist, or altered basic
igneous rocks. The name comes from Cape Emeri in Greece. High-grade deposits
occur on the island of Naxos, Greece, and in Aidin, Turkey. Emery from the island
of Naxos was exploited in ancient Greece. Pliny called emery “naxium”. In the
Cycladic islands the use of emery may have begun in the Neolithic. Emery use in
Crete has been dated to the Middle Bronze Age. The finely detailed sealstones of the
ancient eastern Mediterranean and Near East required the concomitant exploitation
of abrasive raw materials for finishing and polishing.
The vast quantities of finely polished hard nephrite jade artifacts (hardness 6.5)
from the Chinese Neolithic may now be explained by the recent identification that
corundum was worked during the Chinese Neolithic (4000–3500 BCE). These
same corundum rocks later (circa 2500 BCE) may have been given a high polish by
a diamond abrasive (Lu et al. 2005).
10.3 Salt (Halite)
The mineral halite (NaCl, common table salt) has been the source of salt for human
nutritional needs from the beginning of human evolution (Fig. 10.1). It has also
served as a preservative, a medium of exchange, and a significant source of tax
revenue. Salt has been recovered from two major sources: (1) sedimentary deposits
of rock salt, and (2) evaporation of seawater. Halite deposits form when arms of
the sea are cut off from a water supply and the water evaporates. Seawater is about
3.0% NaCl. In coastal estuaries, evaporation can increase this percentage to about
8%. It should be noted that the simple evaporation of seawater to complete dryness
Fig. 10.1 Halite cleavage fragments