Opal:

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.

Opal is considered the birthstone for people born in October or under the sign of Scorpio and Libra.

Middle Ages

The Middle Ages is a period of European history that lasted from the 5th until the 15th centuries. It began with the collapse of the Western Roman Empire, and was followed by the Renaissance and the Age of Discovery. The Middle Ages is the middle period of the traditional division of Western history into Classical, Medieval, and Modern periods. The period is subdivided into the Early Middle Ages, the High Middle Ages, and the Late 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.