Assignment 19 - Opal Flashcards
Opal that lacks play-of- color.
Potch opal (common opal)
What is opal’s main component and internal structure?
Opal’s basic chemistry consists of hydrous silica, so-called because it contains a significant amount of water. There are two broad opal categories: precious and common. Precious opal displays characteristic flashes of rainbow colors, called play-of-color, while common opal does not.
Precious opal’s internal structure consists of an orderly arrangement of uniform, submicroscopic silica spheres.
What properties determine opal’s play-of-color?
Play-of-color is produced by the diffraction of white light when it interacts with precious opal’s orderly internal structure.
What are the theories of opal’s formation process?
Scientists have proposed many different opal formation theories.
The four most representative models are the weathering theory, the microbe theory, the syntectonic theory, and the redox theory. Each one has a different view about opal formation.
How does the weathering theory explain the formation of opal?
Seasonal rainwater soaks the dry ground in opal’s arid source areas. The water dissolves the silica as it trickles downward and deposits it in voids and cracks in the rocks. Much of the water evaporates during the dry season, leaving a solid hydrous silica deposit behind. This hydrous silica deposit is opal.
How does the microbe theory explain the formation of opal?
The microbe theory proposes that the enzymes and acids secreted by tiny organisms, called microbes, slowly dissolve the feldspar and clay minerals in the rock and form a silica-rich gel, which then converts to opal.
How does the redox theory explain the formation of opal?
The redox theory relies on the presence 100 million years ago of a cold, oxygen-poor inland sea that provided the proper environment for opal formation.
How does the syntectonic theory explain the formation of opal?
The syntectonic theory describes a rapid and dynamic process, where silica-saturated solutions are deposited in faults and folds of the host rock.
An opal’s bodycolor, independent of its play- of-color.
Background color
A gem material’s host rock, which sometimes becomes part of the finished gem.
Matrix
What makes black opal the most expensive opal type?
In a black opal play-of-color shows up best against a dark, opaque background.
What is a white opal’s most prized feature?
Almost- transparent gems featuring distinct patterns of crisp, lively play-of-color
How is beauty in boulder opal judged?
Experts judge the beauty of boulder opal by the intensity of its play-of-color and how attractively the matrix is incorporated into the gem.
What makes fire opal suitable for faceting?
Because of fire opal’s clarity and color, it’s one of the few opal types that’s sometimes faceted.
What are the typical components of an assembled opal?
The two common types of assembled opal are the opal doublet and triplet. The doublet is a thin layer of opal cemented to a backing.
The backing is often composed of obsidian, dyed black chalcedony, black glass, natural potch opal, or plastic.
The triplet is a thin layer of opal cemented between a domed top of colorless quartz or clear glass and a backing of obsidian, chalcedony, or black glass.
Thin layers of precious opal, cut to include matrix.
Boulder opal
How is opal pattern categorized?
- Pinfire—Very small patches or “dots” of play-of-color
- Flash—Large areas of play-of-color
- Harlequin—Large, distinct, usually rectangular patches of play-of- color with edges touching each other
What are opal sources in Australia famous for?
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.
What are opal sources in Ethiopia famous for?
Opals mined in Ethiopia are all of ancient volcanic origin.
What are sources in Mexico famous for?
Mexico is the world’s major source of fire opal.
How do you evaluate an opal’s appearance?
- Determine Type
- Play-of-color
- transparency
- clarity
- cut
What pattern is characteristic of most lab-grown opal?
Under magnification, synthetic opal typically shows a structural pattern resembling honeycomb or snakeskin. This tells gemologists that the gem is not natural.
