32 Questions
What is the primary characteristic that distinguishes precious opal from common opal?
Play of color
Which type of opal is considered the most valuable and typically sold by the piece?
Black opal
What is the primary factor that determines the transparency or opacity of opal?
Number of impurities within opal's silica spheres
Which of the following is NOT a characteristic of white opal?
Opaque when held up to light
What is the typical background color of semi-black opal?
Grayish
Which of the following is a characteristic of black crystal opal?
Transparent to semitransparent
What is the purpose of assembling opal layers with other materials?
To improve durability and appearance
What is the characteristic of a triplet opal?
A thin layer of opal cemented between a domed top of colorless quartz or clear glass and a backing of obsidian
What is the chemical composition of opal?
Hydrous silica
According to the weathering theory, how is opal formed?
Through the weathering of rocks by rain water
What is the characteristic of gem-quality non-phenomenal opal?
It occurs in pink, blue, and other colors
What is the characteristic of opal that gives it a picturesque agate-like appearance?
Attractive inclusions of another mineral
What is the primary factor that determines the hue of play-of-color in pink opal?
The size of the silica spheres
What is the term used to describe the vividness of an opal's play-of-color?
Brilliance
What is the most valuable type of opal pattern?
Harlequin
What is the characteristic of an opal that can indicate a lack of stability?
Cloudy or milky background color
What is the requirement for an opal to undergo common treatments such as sugar treatment or smoke treatment?
The opal must be fairly porous
What is the term used to describe the network of cracks that can appear on an opal when it dries out?
Crazing
What is the name of the opal imitation made of glass, which has a similar appearance to play-of-color, caused by tinsel-like flakes?
Slocum Stone
In which country was plastic imitation opal first marketed in the late 1980s?
Japan
What is the primary goal of a cutter when cutting opals?
To maximize the play of color
Which city in India is one of the main fashioning hubs for Ethiopian opal as well as fire opal from Mexico?
Jaipur
What is the purpose of fracture filling in opals?
To correct or prevent crazing
What is the name of the synthetic opal manufactured using chemical precipitation, which has a structural pattern resembling honeycomb or snake skin under magnification?
Gilson Created Opal
Which country was a major cutting hub for opal, but has been replaced by other countries such as India, Vietnam, and Thailand due to economic reasons?
China
What is the typical shape of commercial quality opals?
Oval or round cabochons
During the 1980s-1990s, which country's consumers were the top buyers of opal?
Japan
Which of the following countries is NOT a significant source of opals?
Japan
What is the purpose of coating opals with resin or plastic?
To improve the luster of the opal
What is the term for opals that have been filled with melted wax, resin, polymer, or plastic?
Impregnated
What is the color/hue range for black crystal opal ?
transparent to semi-transparent with a dark background
What are trade names for Opal?
All the above
Study Notes
Opal Basics
- The largest consumers of opal are the Americans, followed by Europe and China, along with other Southeast Asian countries like India.
- Opal has a Mohs hardness of 5 to 7.
Types of Opal
- Precious Opal: Has a play of color, also known as "phenomenal" opal.
- Common Opal: Does not have a play of color, also known as "potch opal".
- Black Opal: Has a dark background, typically the most valuable type, and is usually sold by the piece.
- Semi-black Opal: Has a grayish background, translucent to opaque.
- Black Crystal Opal: Has a dark background, transparent to semitransparent.
- Gray-base Black Opal: Has a gray background, opaque when held up to light.
- White Opal: Has a white or medium gray background, shows play of color.
- Assembled Opal: Layers of precious opal and other material, cemented together to improve durability and appearance.
Color and Hue
- Background Color: Caused by impurities within opal's silica spheres, affects transparency and value.
- Play of Color: Determines the value of an opal, with red being the most preferred hue.
- Multicolors: Play of color consists of one main color and two or more secondary colors.
Patterns and Clarity
- Pattern: The arrangement of an opal's play of color, with industry categories including Pinfire, Flash, and Harlequin.
- Clarity: Opaque to transparent, with value varying depending on type, and cloudiness or milkiness lowering the value.
Chemical Composition and Forming Source
- Chemical Composition: Hydrous silica (SiO2·nH2O), consisting of silicon and oxygen.
- Forming Source: Sedimentary and volcanic rocks, with four recognized theories: weathering, microbe, syntectonic, and redox.
Crystal Structure and Possible Phenomena
- Crystal Structure: No overall crystal structure, but instead a three-dimensional arrangement of uniform, submicroscopic silica spheres.
- Possible Phenomena: Play of color, with the size of the spheres determining the hue displayed.
Treatments and Imitations
- Common Treatments: Sugar, smoke, impregnation, dying, fracture filling, coating.
- Imitation Materials: Slocum Stone, plastic imitation opal, "Opalite", and "Opal Essence".
Cutting and Trading Centers
- Cutting and Trading Centers: Santiago de Querétaro, Mexico, and Jaipur, India, are major centers for opal cutting and trading.
- Historical Trading Hubs: China, Japan, India, Vietnam, and Thailand have been used as cutting and trading hubs.
Other Opal Facts
- Countries of Origin: Australia, Ethiopia, and Mexico are the largest suppliers of opal, with other locations including Brazil, the US, Canada, Honduras, Peru, Madagascar, Slovakia, and Austria.
- Fire Opal: Often found in Mexico, Honduras, and the southwestern United States.
- Trade Names: "Pandora", "Light of the World", and "Empress" are trade names for opal.
Discover the fascinating world of opal! Learn about the largest consumers of opal, its properties on the Mohs scale, and the differences between precious and common opal. Explore the colorful world of opal and its unique characteristics.
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