Opal: Key Concepts , Treatments

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Questions and Answers

What determines the preferred level of transparency in opals?

The type of opal

The size of the opal

The background color of the opal (correct)

The internal structure of the opal

What is the main goal of an opal cutter?

To create a uniform color pattern

To minimize the cost of the opal

To maximize the size of the opal

To maximize play-of-color (correct)

What is unique about fire opal?

It has a uniform black color

It is only found in Australia

It is the rarest type of opal

It is the only type of opal that can be faceted (correct)

What factor does not determining the value of an opal?

The type of opal Signup and view all the answers

What is the purpose of matrix backing in boulder opal?

To make the opal more durable Signup and view all the answers

What is the rarest type of play-of-color in opals?

A range of colors across the entire spectrum Signup and view all the answers

What is the internal structure of precious opal composed of?

Orderly arrangement of uniform, submicroscopic silica spheres Signup and view all the answers

What is the effect of the interaction between white light and precious opal's internal structure?

Diffraction of white light Signup and view all the answers

What is the primary basis for categorizing opal types?

Background color Signup and view all the answers

Which country is NOT one of the most important suppliers of opals in the global market?

Brazil Signup and view all the answers

What is the purpose of assembling opal pieces?

To make attractive finished gems from thin opal pieces Signup and view all the answers

What is the most common type of opal in terms of commercial quality?

White opal Signup and view all the answers

What is the purpose of using sugar and smoke in opal treatment?

To darken the background Signup and view all the answers

What is impregnation in opal treatment?

Filling with melted wax, resin, polymer, or plastic Signup and view all the answers

What is the purpose of fracture filling in opal treatment?

To correct or prevent crazing Signup and view all the answers

What is the purpose of dying as an enhancement treatment for opals?

To change the opal's hue Signup and view all the answers

What is fracture filling, and how is it used to enhance opals?

A type of treatment that fills in fractures with a material Signup and view all the answers

What is the term used when an opal is filled with a colored substance?

Impregnated and dyed Signup and view all the answers

Which type of opals are often just dyed?

Ethiopian opals Signup and view all the answers

What is the purpose of coating as an enhancement treatment for opals?

To protect the opal's surface Signup and view all the answers

What is the purpose of coating an opal with resin or plastic?

To improve the luster Signup and view all the answers

What is impregnation, and how is it used to enhance opals?

A type of treatment that fills in pores with a material Signup and view all the answers

Why might an opal undergo 'crazing' when it is dried out?

Because it has developed a network of cracks Signup and view all the answers

What is the purpose of cementing precious opal layers with other materials?

To improve durability and appearance Signup and view all the answers

What is the top layer of a triplet opal typically made of?

Colorless quartz or clear glass Signup and view all the answers

What is the purpose of using black glass or chalcedony as a backing in assembled opals?

To create a contrasting color background Signup and view all the answers

What is the grading criterion for triplet opals based on?

The opal layer quality and construction Signup and view all the answers

What is the purpose of dyeing chalcedony in assembled opals?

To create a contrasting color background Signup and view all the answers

What is the purpose of using plastic in assembled opals?

To provide a more affordable backing material Signup and view all the answers

What is the range of backgrounds for black opal?

Translucent to opaque black to dark gray, blue, green, or brown Signup and view all the answers

What is the characteristic of a semi-black opal?

Translucent to opaque, with a best example being translucent when held up to light Signup and view all the answers

What is the range of backgrounds for white opal?

Translucent to opaque white to medium gray background Signup and view all the answers

What is the term used to describe the vividness of an opal's play-of-color?

Brilliance Signup and view all the answers

What determines the value of a black opal?

The darker the background, the more valuable Signup and view all the answers

What is the characteristic of a black crystal opal?

Transparent to semitransparent dark background Signup and view all the answers

What is the term used to describe the play-of-color that consists of just one main color and two or more secondary colors?

Multicolors Signup and view all the answers

What is the most preferred hue in opals, followed by orange and then green?

Red Signup and view all the answers

What is the characteristic of a gray-base black opal?

Opaque gray background when held up to light Signup and view all the answers

What is the arrangement of an opal's play-of-color referred to as?

Pattern Signup and view all the answers

What is the industry name of opal that features very small patches or 'dots' of play-of-color?

Pinfire Signup and view all the answers

What is the type of opal where play-of-color is only seen under reflected light?

Contra Luz Signup and view all the answers

What is the typical background color of water opal?

Light in tone Signup and view all the answers

What is the characteristic of the background color of cherry opal?

Transparent red-orange to bright red Signup and view all the answers

What is the range of background color of boulder opal?

Dark or light Signup and view all the answers

What is the characteristic of the background color of crystal opal?

Transparent to semitransparent Signup and view all the answers

What is the characteristic of the background color of fire opal?

Transparent to translucent reds and oranges Signup and view all the answers

What is the range of background color of semi-crystal opal?

Light Signup and view all the answers

Study Notes

Opal Characteristics

Precious opal's internal structure consists of an orderly arrangement of uniform, submicroscopic silica spheres.
Play-of-color is produced by the diffraction of white light when it interacts with precious opal's orderly internal structure.

Opal Types

Black opals: background color ranges from translucent to opaque black to dark gray, blue, green, or brown; shows play-of-color; darker the background, the more valuable.
Semi-black opals: grayish background, translucent to opaque.
Black crystal opal: dark background, transparent to semitransparent.
Gray-base black opal: gray background, opaque when held up to light.
White opal: background color ranges from translucent white to medium gray; shows play-of-color; commercial qualities in pale and cloudy yellows or browns with faint play-of-color.
Crystal opal: background ranges from transparent to semitransparent; shows exceptional play-of-color.
Semi-crystal opal: semitransparent with a light background color.
Water opal: background ranges from transparent to translucent; shows faint play-of-color or no play-of-color; also known as jelly opal.
Boulder opal: includes host-rock fragments or matrix as part of the finished gem; shows play-of-color; thin layers of precious opal cut to include the sandstone or ironstone matrix, making it more durable.
Fire opal: background color ranges from transparent to translucent reds and oranges to yellows; might or might not show play-of-color; color caused by iron oxides impurities; top color is usually red-orange to bright red.

Opal Formation Theories

Weathering theory: rainwater
Microbe theory: enzymes and acids secreted by tiny organisms
Syntectonic theory: silica-saturated solutions deposited
Redox theory: 100 million years ago, a cold, oxygen-poor inland sea

Opal Treatments

Sugar treatment or smoke treatment: darkens the background, making the play of color brighter
Impregnation: fills with melted wax, resin, polymer, or plastic
Dyeing: colors the opal
Fracture filling: corrects or prevents crazing
Coating: improves luster

Opal Market and Sources

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
Black opals command the highest prices
Fire opals are often found in Mexico, Honduras, and in the southwestern United States

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Description

Learn about the internal structure, patterns, and transparency of precious opal, as well as the diﬀraction of light and opal formation theories.