Opal (With Play-Of-Color) | Gemopedia

Opal with play of color has always been considered one of the most desired gems in the marketplace, earning it the undisputed title as Queen of Gemstones. Opal's name evolved from the Greek opallios, meaning to see a change of color. The Greek word was a modification of the ancient Indian Sanskrit name for opal, upala, which meant precious stone. Opal's phenomenal play-of-color effect is not seen in any other gemstone; a unique quality that has attracted the attention of gem lovers through the ages.
Alternate Names
Precious Opal
Colors
Virtually Any Color

Opal (With Play-Of-Color) Classification

Common Name

Opal (With Play-Of-Color)

Species

Opal

Variety

Colors

virtually any color

Alternate Names

precious opal

Gemstone Groups

Key Separations

Key tests are play-of-color, refractive index and fluorescence. Opals might phosphoresce, and this is a strong indication of its natural origins, as synthetic or simulant opals do not phosphoresce.

Comments

Some natural opal is polymer impregnated to make it more stable or improve its appearance.Sugar treated opal will have an RI between 1.440 to 1.450 and a SG between 1.98 to 2.05 both of which are lower than untreated opal. Smoke treated opal typically has an RI between 1.380 to 1.390 and a lower SG than untreated opal. If there is an absence of play-of-color, opal is often referred to as "common opal" instead of "precious opal."Black crystal opal is transparent to semitransparent shows strong play-of-color with a dark bodycolor.Black opal is translucent to opaque with play-of-color against a dark background.Contra-luz opal is a transparent opal that only displays play-of-color in transmitted light.Crystal opal is transparent to semitransparent with a clear bodycolor and strong play-of-color.Jelly or water opal has no color and is transparent to semitransparent and displays weak or no play-of-color.Noble or precious opal is another name for stones that display play of color. Semiblack or gray is translucent to opaque with a light to medium gray background and displays play-of-color.White opal or milky is translucent to semitranslucent on a white background and displays play-of-color.

Opal (With Play-Of-Color) Optical Properties

Transparency

Transparent To Opaque

Refractive Index

1.45 Tolerance: +0.020/-0.080

Birefringence

0-0

Optic Character

NA

Optic Sign

NA

Polariscope Reaction

singly refractive, commonly with ADR (SR-ADR)

Fluorescence

SWUV: Inert to white to moderate light blue, green or yellow
LWUV: Inert to white to moderate light blue, green or yellow

CCF Reaction

Pleochroism

Dispersion

Strength: none

Comments

Opal (With Play-Of-Color) Chemistry & Crystallography

Chemical Name

silica

Chemical Formula

SiO2.nH2O

Synthesis

Crystal System

Classification

Silicate

Nature

Natural

Crystallinity

Polycrystalline

Comments

Opal (With Play-Of-Color) Characteristic Physical Properties

Hardness

6

Streak

white

Specific Gravity

1.25-2.23 Range: +0.08/-0.90

Toughness

Poor To Fair

Inclusions

Opal will display play-of-color in various patterns, such as pinfire (small patches or dots), flash (large areas of color), harlequin (brush-strokes of color); flagstone and other unique patterns. Sugar treated opal will show black specks in-between the areas of play of color under magnification. Smoked stones will have black specks in its pores when viewed with magnification and will have dark brown to black surface coloration. If sugar treated or smoked stones are chipped the colorless layer might be exposed.

Luster

Vitreous To Resinous

Stability

Poor To Fair

Fracture

Conchoidal To Uneven

Cleavage

None

Comments

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