Opal is a relative of quartz but is itself not a mineral, it is a mineraloid due to it’s lack of crystalisation. 97% of opal is mined in Australia, where it is the national gemstone. Prized for their ability to display every colour of the visible light spectrum.
The reason for opal’s incredible refraction ability is due to it’s structure. Silica spheres arranged in planes, or rows, that taper to a pyramid shape, the spacing and posistioning of the planes, in response to the angle of refraction determined by the appropriate angle of incidence, that causes the dispersion of light leading to incredible refraction. To give you an idea of how well light is refracted in opal, opal’s refractive index is 1.45 which means that in a vacuum light travels 1.45 times faster than it does in opal, (as a point of reference, water’s refractive index is 1.33).
Clarity is difficuilt to measure in opal due to the extreme commonality of ‘veins’ within opal which instead of being seen as a disadvantage are essential to the distinctive light performance that makes opal’s so sought after. Instead opal’s are ranked either translucent, semi-translucent or opaque. Opal’s are often treated, commonly with oils, and this is one of the main reasons that they cannot be placed in an ultrasonic cleaner. The other is the porous quality of opal (some opal’s can be 30% water), means that the cavatation process used in ultrasonic cleaning would shatter the opal.
In terms of cut, a cabachon cut is usually used, to avoid abrasion and enable the best range of light play through the opal. Although this can still be used in it is different to the usual apperance. But that could be perfect.