Opal (Common) | Gemopedia

While we often think of opal in terms of phenomenal play of color, this gemstone family is full of other unique and appealing members that have their own allure. Gems can be transparent, translucent or opaque and form in almost any color in many locations around the world. Get to know all the varieties of this unique family by looking at our fire opal, dendritic opal, hyalite, Caramel Spice Opal (TM), Morado Opal (TM), along with even more blue, pink and green varieties. With such an endless array, opal truly is the 'Queen of Gemstones'.
Alternate
Names
Potch, Opal Without Play-Of-Color, Common Opal, Colored Opal
Colors
Almost Any Color

Opal (Common) Classification

Common Name

Opal (Common)

Species

Opal

Variety

Common Opal

Colors

Almost Any Color

Alternate Names

Potch, Opal Without Play-Of-Color, Common Opal, Colored Opal

Gemstone Groups

Key Separations

Appearance, RI and fluorescence

Comments

Common opal shows no play-of-color and is called potch when found in association with precious opal. Agate opal or banded opal has concentric bands and has an appearance similar to agate. Andean opal is a greenish blue opal from Peru that sometimes contains dendrite inclusions. Cachalong opal, mother of pearl opal or Kalmuck agate opal is translucent to opaque milky bluish white and it has a similar appearance to porcelain or mother of pearl. This stone is often used form cameos. Cat's-eye opal is brownish yellow to yellowish brown and has needle inclusions that create chatoyancy. The stones might display what is called the "milk and honey" effect. Dendritic or moss opal has dendritic inclusions that look like moss. Girasol opal is a hyalite opal that is semitransparent to translucent. It has microscopic inclusions that give it blueish white sheen with reddish reflections and has an appearance similar to moonstone. Hyalite opal or water stone is colorless to translucent white with strong sheen. Honey opal is orange brown in color. Iron opal is yellow or red common opal.Jasper opal is brecciated Jasper cemented by opal. Milk opal is white common opal.Onyx opal is banded opal. Prase opal or chrysopal is a translucent to opaque green common opal. The yellowish green material looks similar to chrysoprase. Wax opal is a yellowish in color and has a waxy luster. Wood opal or zeasite is dark brown to black opaque fossilized wood replaced by opal.

Opal (Common) Optical Properties

Transparency

Translucent - Opaque

Refractive Index

1.37-1.47

Birefringence

Optic Character

NA

Optic Sign

Polariscope Reaction

Singly Refractive (SR) With ADR

Fluorescence

SWUV: inert to strong green or yellowish green
LWUV: inert to strong green or yellowish green

CCF Reaction

Pleochroism

None

Dispersion

Comments

Opal (Common) Chemistry & Crystallography

Chemical Name

hydrated silicon dioxide

Chemical Formula

SiO2.nH2O

Synthesis

Crystal System

NA

Classification

Silicate

Nature

Natural

Crystallinity

Amorphous

Comments

Opal (Common) Characteristic Physical Properties

Hardness

5-6.5

Streak

White

Specific Gravity

1.98-2.5 Typical:2.15

Toughness

Varies

Inclusions

Luster

Vitreous

Stability

Poor

Fracture

Conchoidal, Uneven

Cleavage

None

Comments

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