What makes gems so special

SPAZIO CLIL
What makes gems so special
N
o one can be sure when the first human picked
up a mineral crystal and kept it for its rare
beauty, but we do know that gems were being
worn as necklaces and other adornments at the
dawn of civilization in Egypt, at least 4000 years
ago. These early Egyptians were undoubtedly attracted to the color and play of light on the polished surfaces of such minerals as carnelian, lapis
lazuli, and turquoise. Color and luster (the ability
to reflect light) are two qualities that still serve to
define gemstones. Although the value placed on
a gemstone varies in different cultures and historical periods, other required qualities seem to be
beauty, transparency, brilliance, durability, and
rarity. Most minerals have some of these remarkable qualities, but the stones considered most precious are ruby; sapphire; emerald; and, of course,
diamond.
A diamond (geologically speaking, at least) may
not be forever, as the advertisers claim, but it is
special. Its glitter is unique, as are the play of colors
and the sparkle that it emits. The source of these
qualities is the way in which diamond refracts, or
bends, light. These characteristics are enhanced
by diamond’s remarkable ability to split perfectly
along certain directions of the crystal, which diamond cutters use to advantage in carefully cutting
gem-quality stones. Diamond’s multiple facets
(faces superficially similar to crystal faces) can
be polished to enhance this sparkle. These facets
can be ground only by other diamonds, because
diamond is the hardest mineral known, so hard
that it can scratch any other mineral and remain
undamaged. This mineral’s tightly packed crystal structure and strong covalent bonds between
carbon atoms give it these characteristics, which
allow it to be identified with certainty by mineralogists and jewelers.
Rubies and sapphires are gem-quality varieties of the common mineral corundum (alumi-
Fantini, Monesi, Piazzini - Elementi
num oxide), which is widespread and abundant
in a number of rock types. Although not as hard
as diamond, corundum is extremely hard. Small
amounts of impurities produce the intense colors
that we value. Ruby, for example, is red because of
small amounts of chromium, the same substance
that gives emeralds their green color.
Less valuable, sometimes called semiprecious,
gemstones are topaz, garnet, tourmaline, jade, turquoise, and zircon. Most, like garnet, are common
constituents of rocks, occurring mostly as small
imperfect crystals with many impurities and poor
transparency. But, under special conditions, gemquality garnets form. From time to time, some
minerals that are not ordinarily considered gems
may enjoy sudden (perhaps temporary) popularity. Hematite (iron oxide) currently enjoys this status, appearing in necklaces and bracelets.
Ring with sapphire (blue) and diamond (colorless).
di Scienze della Terra • Italo Bovolenta editore - 2013
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