Play of color - fire in Opal
I hope this helps more than it confuses. I am far from expert in this area and have relied heavily upon the work of others, but I have tried to present the information in a way that can be understood by the layperson. Alot of the old beliefs still persist in explaining the fire in opal and much of what you "know" may be contradicted herein. I assure you that this represents the latest scientific fact in this area and is indisputable, having been derived from rock solid data (pun intended).
First some pictures to illustrate the terms we use.[PLACE CURSOR OVER PICTURE FOR INFORMATION].
Now some terms ...
Aventurescence-sparkling, glittering effect from reflections of light off tiny platelets of (usually)copper, hematite, or mica.
Andularescence- a pearly floating light effect which glides over the surface caused by structural factors- applied to the effects found in moonstone.
Asterism- light rays forming a star - caused by reflections off thin needle inclusions which cross each other.
Chatoyancy- a band of reflection resembling a cat's eye - actually the same phenomenon as in Asterism, but the needles are arranged parallel in the stone.
Opalescence- the soft sheen of common opal, caused by reflection of mostly blue short-wave light. This is NOT play of color.
Play of Color- we will examine this on this page. It is unique to opal and is caused by microscopic spheres within the opal.
Iridescence- a lustrous play of changing rainbow colors as found in fire agate and ammolite. It is caused by cracks or structual layers causing light to break into its spectral colors. The result can be very close to the appearance of play of color in opal.
Now we get to the point! Opal is pretty much an amorphous material - hydrated silicon dioxide (which makes it a species of Quartz!). It always has a water content, but that may range from 3% to as high as 20%(usually less than 10%). Amorphous means that it has no long-range order of atoms; crystals DO have a long-range atomic order. For a long time this caused much confusion as to where the play of color came from.
Even though opal does not have a crystal structure, it does have structure. Very tiny spheres of silicon and oxygen are chained randomly in irregular size and concentration. In Precious Opal (opal with play of color) there are areas where the spheres are organized. The density and pattern of these organized/aligned spheres are responsible for the play of color. But we jumped ahead - you see, amorphous materials should not, cannot create a play of color, so far many years all kinds of postulations presented all kinds of reasons for the color.
Around 1900 a fella named Mallard hit on the right path, but was largely ignored. Along came X-ray diffraction and Transmission Electron Microscopy and a treak down the right path until, finally in 1955 the correct answer was presented and pretty much elaborated on in 1963 to its present state.
Within precious opal are small growths of cristobalite and tridymite crystal layers. Opal is now classed as follows:
Opal-A: Very amorphous structure. Opal-AG envelopes Hyalite (glass-like opal mainly associated with volcanic rock areas); 3% water, structure similar to silica glass; most precious opal is Opal-AG. Opal-AN envelopesopal from sedimentary fields, biogenic silicas from plants or primative animal life; 5-10% water, structure similar to silica gels.
Opal-CT- contain disordered interlayers of short-range ordering of cristobalite and tridymite structures of small crystallite size, occuring in the disordered opal structure.
Opal-C- a small group associated with volcanic rock which is has considerably more short-range order than Opal-CT, nearly 50% of the material may lay in regions structured similarly to low critobalite.
All of which is way more than one needs to know and is highly summarized from a variety of fields. The purpose is to give you some understanding of the nature of this wonderful material. That may help us in working with it.
The main points: opal does and MUST contain water. Opal crazes when the water is leached out. So long as the opal is kept in water, a homeostasis exists and crazing will not occur until the opal is removed from the water - then it may be 6 months or a year before the tragic event begins. This is the reason we do not store our opal in water - if it is going to craze, let it be before you buy it. The idea that polishing seals the surface is hogwash - the electron microscope also put that tale to death.
The play of color in precious opal is caused by somewhat ordered layers of tiny spherical crystals of cristoballite and tridymite layered in small regions within the disorganzed material of opal. The size and structure of these tiny layers give the variety of fire within the gem.
So what? So ... that is why the fire in opal most always appears from within, and why when you bring it to the surface it can suddenly be sanded away in a moment. In opal, when the fire does seems to shine right on the surface, the cutter has succeeded in completing his polish just as the very tiny crystallite layer is at the fore - no minor achievement.
-- Click here to email us ! -- 
Return Home