What is opal?
Opal is a hydrated amorphous form of silica.
The chemical formula for opal is: SiO2 n H2O
Opal has a hardness rating of 5.5 to 6.5 on Moh’s Scale – a scale that measures resistance to scratching. (On Moh’s scale diamond is 10 and talc is 1).
Because opal is not a hard stone it is more suited to jewelry designed for dress wear. In light of this an opal engagement ring should be set in a bezel and worn with care. Learn more about opal engagement rings here.
How is opal color produced?
Opal color is produced by the internal arrangement of silica spheres.
The fundamental component of opal are silica spheres.
Silica spheres in opal vary in size.
Large spheres produce red opal color; small spheres produce purple opal color.
Two Types of Opal
There are two types of opal: Precious opal and Common opal
‘Potch’ is another name for common opal.
Both types of opal are made of the same chemical components, silica and water, however their spherical arrangement differs.
Precious opal is composed of microscopic silica spheres packed uniformly in an orderly network. The spaces between the spheres contain silica in solution and it is this solution that deflects the light, once it has passed through the translucent silica sphere, back out through that same sphere, only this time producing colour. The opal color produced depend entirely on the size (and arrangement) of the spheres (Cody, 1991). Small spheres produce purples, large spheres produce reds. Purple is more common than red (Ratliff, 2014).
Common opal, or potch, is composed of spheres that are erratically arranged. Because of this chaotic arrangement common opal does not produce complex opal colors or ‘play of color.’
FAQ: What is the rarest opal color?
The rarest opal color is Red.
Red opal color is produced by large spheres which do not occur as often as small spheres. Small spheres occur more frequently and produce the opal colors blue and purple.
Red opal color is highly prized and very rare.
‘Black’ color is more rare than light color.
‘Black’ color is a grading term used to describe the darkness or depth of color.
‘Black’ red or ‘Black’ green or ‘Black’ blue are rare opal colors with high value.
If the red opal color presents in complex patterns e.g. pinfire or harlequin pattern, this increases the value of the opal according to the rarity of the color, pattern and size.