Continued from page 1
Matrix opal is when opal is infused in rock in which it was formed. Veins of precious opal permeate sandstone or ironstone filling holes and crevices of host rock.
Synthetic opal as name suggests, is produced in laboratory and has a similar structure to that of precious opal.
The following observations can be made to differentiate between natural and synthetic opal:
a. Synthetic stones show brighter colours and larger colour patches;
b. In synthetic opal, colour grain boundaries are highly irregular;
c. Synthetic opal has a distinctive snakeskin patter;
d. The material shows a more ordered array of colour because intricate patterns of natural opal cannot be duplicated.
Rough Opal Parcels:
Rough opal parcels are sorted into three grades: Top, Middle and Low.
Each Top parcel has a King Stone (the best stone in parcel). Colour is primary criteria for grading.
You can buy rough opal in several different conditions:
a. Mine Run – Directly from miner himself. These stones have not yet been cut or graded.
b. Off Cuts – The miner has removed all marketable opal and you are sold what is left. With off cuts, you can usually tell what you are going to be able to cut.
c. Rubs – The opal stones have been cut and ground into basic shapes, after having removed most of rubbish.
A more porous opal found in mining fields. Out of ground it is quite pale, but after treating it with a carbon dye process it looks like a real, black opal.
This rare and unusual opal, up to 5 cm across, looks like a round or oval nut and is an ironstone formation.
Opal forms within nut either as a solid kernel or more commonly, in concentric layers infused with host rock. The nut has to be split open before contents are revealed and it may be split so that mirror images are produced.
Opal doublets, as name suggest, is a thin layer of precious opal cemented onto a backing, usually a piece of black common opal or ironstone. Produced because opal is too thin to produce a solid stone, doublets are a cheap and inexpensive way to buy a colourful piece of precious opal without cost involved of a solid.
Similar to a doublet, but has three pieces sandwiched together, with a thin piece of precious opal cemented to a backing and a clear quartz cap placed on top to protect and bring out colour of opal.
That conlcludes part 5. In final instlament I have combined Parts 6 and 7. We learn how opal is valued and I have included a dictionary giving definitions of those terms and words which you may be unfamiliar with.
I hope you have enjoyed reading this article and as always, I look forward to seeing you all again soon.
Best wishes and have a great day