Chemistry Reference
In-Depth Information
Ruby and Sapphire.
Ruby
and
sapphire
are “sister stones”: both are gem-
stone forms of the mineral
corundum
(composed of aluminum oxide). Pure
corundum is colorless, but a variety of trace elements cause corundum to
exhibit different colors. Ruby is red corundum, while sapphire is corundum
in all colors except red. The red in rubies is caused by trace amounts of
chromium; the more intense the red color of a ruby, the more chromium it
contains. The blue in
sapphires
is caused by titanium and/or iron impurities
(Garland 2002; Hughes 1997).
Turquoise.
Turquoise
is an opaque, usually blue, mineral (composed of
hydrated aluminum copper phosphate) that has been widely prized as a
semiprecious gemstone for well over 8000 years, having been used to make
beads, rings, and other decorative objects. Deposits of turquoise are formed
when water flows and dissolves part of rocks rich in aluminum and then
evaporates. The blue color of the stone is due to the copper; iron impurities
give rise to shades of blue-green and even to green turquoise. Turquoise
deposits occur in only a few and limited arid regions of the world, such as
the Sinai Peninsula in the Middle East and the southwest of North America.
There seems to have been a wide network of transportation and trade of
turquoise from the places where turquoise was extracted to where it was
worked and eventually used. Identifying the distribution routes of ancient
turquoise has attracted much attention, and a number of investigations have
been centered on establishing the distribution of turquoise from various
mining regions. In one such study, concerned with turquoise beads in North
America, differences in the nature and concentration of impurities in the
gemstone were used to distinguish between turquoise from pre-Columbian
sites in the midwest of the United States and some parts of Mexico. Most of
the turquoise analyzed seems to have originated from mines in the south-
west of North America, from where it was distributed, through well-
established trade routes, to the workshops where it was worked (Harbottle
and Weigard 1992; Sigleo 1975).
