What is Black Opal

Composition of Black Opal

Without going into a college degree for Mineral Science opal is Hydrous Silica with the chemical formula of SiO2•n(H2O). Opal is also known as Amorphous, that is it has no defined crystalline shape. Notice the common (H20) in the formula, yes you have it. Opal has water in it, typically around 6% to 10% in precious Black Opal. This can range from 3% to 21% in many cases.

Crazing in Black Opal

Black Opal
Water in fact is so important to opal that if the opal dries out in can crack or "craze".  This cracking or "crazing" can destroy the opals worth and hence many uncut Black Opals are stored in water to prevent this happening prior to cutting and polishing. The gems are then carefully dried and stabilized over time and together the polishing process helps preserve the opal. The vibration of the cutting and polishing machines themselves can cause the opal to crack as well. The location where the opal is found also depends on its predisposition to crazing with some areas well known for cracking.

As jewelry Black Opals love to be worn often and next to the skin as it helps preserve the moisture within the gem.

Play of Color in Black Opal

What differentiates Common Opal from Precious Opal is what is termed "Play of Color". This means the display of lustrous colors of the rainbow that change and alter with different viewing angles. "Play of Color" is produced by the diffraction of white light through a micro-structure of orderly arrayed spheres of the silica.
The small spheres produce blue color whilst the larger spheres display reds. This can be seen only with the assistance of a powerful electron microscope.

Black Opal
Micro-structure of precious opal
Common Opal or "Potch" is opal that does not exhibit a "Play of Color". "Potch" has no order in its micro-structure and therefore produces no color.

Black Opal
Micro-structure of common opal
Black Opal is characterized by a strong "Play of Color" against a dark background.

Patterns of Color in Black Opal

The patterns of color in Black Opal are an important part of its charm, the patterns can be characterized by some of the following terms.

  • Flash         A broad irregular flashes of colour which change or disappear as the stone is rotated.
  • Harlequin A patchwork of irregular sized squares of colour.
  • Pinfire       Closely spaced pinpoints or specks of colour.

Body Tone of Black Opal

The background of Black Opal when viewed face up is referred to as "Body Tone".
On the scale below Black Opal is considered to have a "Body Tone" from N1 to N4.

Black Opal

N5 to N6 is classed as Dark Opal. N7 to N9 is classed as Light Opal.

Brightness of Black Opal

The brightness of opals is rated 1 to 5 with 1 being dull and 5 the brightest. It is generally advised when purchasing Black Opal that the brightness score be at least 3.

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