Opal is an
amorphous form of
silica , a
mineraloid form, not a mineral. 3% to 21% of the total weight is
water,
but the content is usually between 6% to 10%. It is deposited at a
relatively low temperature and may occur in the fissures of almost any
kind of
rock, being most commonly found with
limonite,
sandstone,
rhyolite,
marl and
basalt. Opal is the national gemstone of
Australia, which produces 97% of the world's supply.
[4] This includes the production of the state of
South Australia, which amounts to around 80% of the world's supply.
Opal's internal structure makes it
diffract
light; depending on the conditions in which it formed it can take on
many colors. Opal ranges from clear through white, gray, red, orange,
yellow, green, blue, magenta, rose, pink, slate, olive, brown, and
black. Of these hues, the reds against black are the most rare, whereas
white and greens are the most common. It varies in optical density from
opaque to semi-transparent. For
gemstone use, its natural color is often enhanced by placing thin layers of opal on a darker underlying stone, like
basalt.
Precious opal shows a variable interplay of internal colors and even
though it is a mineraloid, it has an internal structure. At micro scales
precious opal is composed of silica spheres some 150 to 300
nm in diameter in a hexagonal or cubic
close-packed lattice. These ordered silica spheres produce the internal colors by causing the
interference and
diffraction of light passing through the microstructure of the opal.
[6]
It is the regularity of the sizes and the packing of these spheres that
determines the quality of precious opal. Where the distance between the
regularly packed planes of spheres is approximately half the wavelength
of a component of
visible light, the light of that wavelength may be subject to diffraction from the
grating
created by the stacked planes. The spacing between the planes and the
orientation of planes with respect to the incident light determines the
colors observed. The process can be described by
Bragg's Law of diffraction.
Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical
band gap in a
photonic crystal,
of which opal is the best known natural example. In addition,
microfractures may be filled with secondary silica and form thin
lamellae inside the opal during solidification. The term
opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed
play of color. Contrarily,
opalescence is correctly applied to the milky,
turbid appearance of common or
potch opal. Potch does not show a play of color.
The veins of opal displaying the play of color are often quite thin,
and this has given rise to unusual methods of preparing the stone as a
gem. An opal
doublet is a thin layer of opal, backed by a swart mineral such as
ironstone,
basalt, or
obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch.
Combined with modern techniques of polishing, doublet opal produces
similar effect of black or boulder opals at a mere fraction of the
price. Doublet opal also has the added benefit of having genuine opal as
the top visible and touchable layer, unlike triplet opals.
The triplet-cut opal backs the colored material with a dark backing, and then has a domed cap of clear
quartz
or plastic on top, which takes a high polish and acts as a protective
layer for the opal. The top layer also acts as a magnifier, to emphasize
the play of color of the opal beneath, which is often of lower quality.
Triplet opals therefore have a more artificial appearance, and are not
classed as precious opal.
Superstitions of Opal
In the
Middle Ages, opal was considered a stone that could provide
great luck because it was believed to possess all the virtues of each
gemstone whose color was represented in the color spectrum of the opal.
It was also said to confer the power of invisibility if wrapped in a fresh
bay leaf and held in the hand.
Following the publication of Sir
Walter Scott's
Anne of Geierstein in 1829, however, opal acquired a less auspicious reputation. In Scott's novel, the Baroness of Arnheim wears an opal
talisman with supernatural powers. When a drop of
holy water
falls on the talisman, the opal turns into a colorless stone and the
Baroness dies soon thereafter. Due to the popularity of Scott's novel,
people began to associate opals with bad luck and death.
]
Within a year of the publishing of Scott's novel in April 1829, the
sale of opals in Europe dropped by 50%, and remained low for the next
twenty years or so.
Even as recently as the beginning of the 20th century, it was
believed that when a Russian saw an opal among other goods offered for
sale, he or she should not buy anything more since the opal was believed
to embody the
evil eye.
Middle Ages
In the Early Middle Ages, depopulation, deurbanization, and
barbarian invasions, which began in
Late Antiquity, continued. The barbarian invaders formed new kingdoms in the remains of the Western Roman Empire. In the 7th century
North Africa and the
Middle East, once part of the Eastern Roman Empire (the
Byzantine Empire), became an
Islamic Empire after conquest by
Muhammad's
successors. Although there were substantial changes in society and political structures, the break with
Antiquity was not complete. The still sizeable Byzantine Empire survived and remained a major power. The empire's law code, the
Code of Justinian,
was widely admired. In the West, most kingdoms incorporated extant
Roman institutions, while monasteries were founded as Christianity
expanded in western Europe. The
Franks, under the
Carolingian dynasty, established an empire covering much of western Europe; the
Carolingian Empire endured until the 9th century, when it succumbed to the pressures of invasion — the
Vikings from the north; the
Magyars from the east, and the
Saracens from the south.
During the High Middle Ages, which began after AD 1000, the
population of Europe increased greatly as technological and agricultural
innovations allowed trade to flourish and crop yields to increase.
Manorialism — the organization of peasants into villages that owed rent and labor services to the
nobles; and
feudalism — the political structure whereby
knights and lower-status nobles owed military service to their overlords, in return for the right to rent from lands and
manors - were two of the ways society was organized in the High Middle Ages. The
Crusades, first preached in 1095, were military attempts, by western European Christians, to regain control of the Middle Eastern
Holy Land from the
Muslims. Kings became the heads of centralized nation states, reducing crime and violence but making the ideal of a unified
Christendom more distant. Intellectual life was marked by
scholasticism, a philosophy which emphasized joining faith to reason, and by the founding of
universities. The mathematics of
Fibonacci and
Oresme, the philosophy of
Thomas Aquinas, the paintings of
Giotto, the poetry of
Dante and
Chaucer, the travels of
Marco Polo, and the architecture of
Gothic cathedrals such as
Chartres are among the outstanding achievements of this period.
The Late Middle Ages were marked by difficulties and calamities, such
as famine, plague, and war, which much diminished the population of
western Europe; in the four years from 1347 through 1350, the
Black Death killed approximately a third of the European population. Controversy,
heresy, and
schism
within the Church paralleled the warfare between states, the civil war,
and peasant revolts occurring in the kingdoms. Cultural and
technological developments transformed European society, concluding the
Late Middle Ages and beginning the
Early Modern period.
