Opal, also called noble opal in mineralogy, is a kind of silica which is hydrous and non-crystalline and its chemical formula is SiO2•nH2O. The water content is usually 3% - 10%.

Opal deserves a seat in the table of gemstone kingdom for its beautiful and charming changing colors and its unique attraction. It has the flame of the ruby, the bright purple of the amethyst and the jade green of the emerald, combining the gorgeousness of ruby, the grace of amethyst and the charming of emerald. Opal is regarded as the symbol of hope and purity. The artist once used such poetic words and phrases to describe the colors of the opal: “After the God of nature decorates the flowers, colors the rainbow, dyes the wings of the birds, he pours all the rest pigment into the opal.” Then how do the changing colors of opal form? In the mid-60s of 20th century, scientists used electron microscope to study the opal and found that the colors of the opal rated to its inside structure. The inner structure of opal is composed of numerous of optical transparent silica balls which are arranged regularly. When the light goes through the holes among these balls, it will cause the diffraction. As we all know, the balls and holes are arranged regularly in the three- dimensional space and the size of the ball is the same as that of the hole, so the three-dimensional diffraction framework is formed. The diffraction index changes along with the size of the ball. If the diffraction index is different, the wave length of the diffraction light is also different. That is to say, the change of the size of the ball directly affects that of the wave length of diffraction and causes the change of the angle of diffraction and take on different colors seeing from different angles and that causes the flowery changing colors. The opal composed of small balls, the wave length of diffraction is short, taking on purple color changing; if composed of big balls, the length is long, taking on red color changing. If the sizes of the balls are quite irregular, the arrangement in the three-dimensional space will be irregular. Due to the interference of light, it will not cause the colorful diffraction light, it will only take on the dull milky white and form the common opal and it will not that attractive as the gemstone.

The opal has glass luster, from semi-transparent to non-transparent and the transparent opal is rare. Its refractive index is 1.40 – 1.45, no birefringence phenomenon, the dispersion is very weak. The Mohs hardness is 5.5 – 6.5, the relative density is 2.0 – 2.2 and the fracture is conchoidal. Due to the dehydration of the water contained in the opal, it causes cracks and break easily, so we need to avoid it from high temperature and strike.

The opal has been popular with people due to its colorful changing colors. But different people in different countries and districts have different interests toward the hues of opal. Most Americans like red and orange red opal, this hue is impassioned and has dynamic feeling, which fits the mentality of westerners very much. The Japanese usually likes the blue and green opal because it gives them the feeling of quietness and calmness, which no doubt is the wish of seeking quietness in movement for the nervous and busy Japanese. The Chinese people has always

Opal is regarded as the symbol of hope and purity in western countries, and the eastern considers it as the holy gemstone representing loyalty. It is also taken as the birthstone of October.

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