CS Book 4_AS- 19 Opal .pdf

Colored Stones 19 KEY CONCEPTS Precious opal’s internal structure consists of an orderly arrangement of uniform, submicroscopic silica spheres. In general, opal experts prefer patterns of large, closely arranged color patches. Play-of-color is produced by the diffraction of white light when it intera...

Colored Stones 19 KEY CONCEPTS Precious opal’s internal structure consists of an orderly arrangement of uniform, submicroscopic silica spheres. In general, opal experts prefer patterns of large, closely arranged color patches. Play-of-color is produced by the diffraction of white light when it interacts with precious opal’s orderly internal structure. The preferred level of transparency depends on the background color of the opal. Scientists have proposed many different opal formation theories. Cutters typically process commercial-quality opals into standard sizes or as potential components for assembled stones. Opal types are often based on background color. Maximizing play-of-color is an opal cutter’s main goal. Black opals command the highest prices of all opal types. Australia, Ethiopia, and Mexico are the global opal market’s most important suppliers. White opal makes up the bulk of commercial-quality opal on the market. Matrix backing can make boulder opal more durable than other opal types. The most valuable fire opals usually display uniform redorange to bright red color. Fire opal is one of the few opal types that might be faceted. Assembled opal makes it possible to make attractive finished gems from thin opal pieces. Color, pattern, and clarity combine to create an opal’s unique beauty. The rarest play-of-color contains hues that range across the entire spectrum. Red is often considered the most desirable play-of-color hue. 226 Australia boasts rich deposits of black, white, crystal, and boulder opal. The world’s main source of fine-quality black opal is the New South Wales town of Lightning Ridge. The world’s main source of boulder opal is the Australian state of Queensland. Opals mined in Ethiopia are all of ancient volcanic origin. Mexico is the world’s major source of fire opal. China and India are two major developing markets for opal. Jaipur is a major cutting center for Ethiopian opal. Try to keep conditions, methods, and samples consistent when you evaluate opal.