Black Opal is solid opal distinguished by the black or very dark body colour of the stone and is highly sought after because of its beauty and rarity. Semi Black opal has a background which ranges from smoky grey to nearly black.

The dark backgound accentuates the play of colour in the opal and the brighter and sharper the colours, the more valuable the opal.

Most Black Opal comes from Lightning Ridge in New South Wales or Mintabie with small amount coming from the fields around Coober Pedy. See our Black Opal page for more detail.

White or light opal is solid opal distinguished by a white or light background which is opaque. The play of colour is generally more subdued than black opal however, quality white opal can be brilliant and display many colours.

White Opal generally comes from Coober Pedy and Mintabie in South Australia. See our White Opal Page for more detail.

Transparent or translucent opal is defined as crystal and here the colors are sharp and visible below the surface. Crystal opals come in all color variations and display bright colors. Crystal opals can be either light or dark and are solid.

Crystal opal is clear enough to see through against a light surface but when viewed on a dark surface the colours really spring to life. In terms of value, the more transparent the stone is, the more it is worth. See our Crystal Opal page for more detail.

The term "fire opal" is commonly used to describe the clear orange crystal opals which come from Mexico, some of which have a play of colour. Many have just an orange or reddish base with no play of colour and most of these stones are faceted.

The term "fire" is also used to describe the dominant colour reflected or diffracted out of any opal.

Mined at Yowah in western Queensland Yowah nuts are opal deposits which have formed inside a nut like kernal of ironstone.

They are usually very hard and can be polished to a very high gloss and are often made into attractive jewelry.

Andamooka matrix is a very porous opal which when mined is light in colour. It is cut and polished then carbonised by treating it with sugar and sulphuric acid. The end result is a stone which looks very much like good black opal. It has a characteristic salt and pepper look and can be easily identified when looked at under a magnifying glass.

Matrix opal can also be formed naturally where the silica has run into the minute cracks and crazing in ironstone boulder and this form is known as boulder matrix

Boulder opal is a naturally formed solid opal which consists of a fine layer of opal that has been deposited by nature on and in the fissures of ironstone base rock. Because it geneally has a black or very dark background colour it has the appearance of black opal.

There are often inclusions in the opal layer and because the layer follows the contours of natural ironstone the shape is often very undulated. Depending on how the stone is polished it can either be very smooth or be "pockmarked" where ironstone protrudes to the surface. The irregular shape of boulder opal makes it a designers delight and most pieces of jewelry using boulder opal are very unusual.

Boulder opal is found at a number of locations in Queensland with some small quantities coming from Andamooka in South Australia.

Doublet Opals consist of a layer or slice of natural opal which has been bonded to a dark or black backing. The backing can be potch, black glass or ironstone boulder. These days most doublets are bonded to an ironstone backing thus forming a "boulder doublet". The dark backing enhances the colours and has the effect of simulating high quality black opal. However because less opal material is required doublets are much cheaper.

Opal has existed for many millions of years and it is thought that the the name opal is derived from the Greek "Opallus" which means to see a change in colour.

Precious opal is very rare and is found in only a few locations in the world because it requires very special conditions in order to form. We are very fortunate that this did happen on a such large scale in Australia which now produces over 95% of the world's precious opal.

Approximately 140 million years ago much of central Australia was an inland sea and fine mineral sands rich in Silica were washed up on its shores. The sea eventually receded and formed what is now kown as the Great Artesian Basin. Around 30-40 million years ago, heavy weathering disolved some of this silica and formed a gel solution. This gel then seeped into crevices and cracks in the ground and over many more millions of years eventually hardened, forming what we call opal.

Most of the opal formed was common opal or "potch" which is opal that does not have a play of colour. A very small percentage formed as precious opal which displays the "play of colour" or the prismatic colours of the rainbow that we all know and admire. The difference is not caused by impurities but by the arrangement of the spheres of silica which make up the opal.

Precious opal consists of uniform spheres of silica which form a 3D grid. As white light hits the silica in solution between these spheres it is diffracted and reflected back as different colours, hence the "play of colours". The size and spacing of the spheres determines the dominant colour of the stone. The brilliance of the stone is determined by the uniformity of the spheres and their spacing.

Common opal on the other hand, is made up of silica particles or spheres that have random sizes and spacing which disperse the incident light and therefore have no play of colour.

If we get technical, opal is the gel form of hydrous silicon dioxide, with a chemical formula of SiO2nH2O. It has a hardness of 5.5 to 6.5 which is similar to garnet and has a specific gravity of 1.95 to 2.2. It is a member of the quartz family and has the same chemical composition, however quartz is the crystalline form.

Opal has been adopted as the national gemstone of Australia and is the birthstone for May and October.

Valuing Opal
Opals are unique and no two are exactly alike. This makes valuation a complex process, however some of the important factors in choosing and valuing an opal are:

Type
There are many types - common are solid, boulder, matrix, crystal etc. Solid opal, particularly black is the most rare and valuable however other factors do heavily influence value.

Brilliance or Brightness
This is the brightness of "fire" coming from the opal and does not refer to the base colour of the opal. In order red is the most valuable with orange, yellow, green, blue and violet following.

Pattern
Good patterns of colours combined with brilliance have a huge effect on value. Broad patterns are more desirable than small or pinfire patterns and distinct patterns such as harlequin, straw, ribbon and rolling flash are very rare.

Base Colour
This is the background colour of the stone. When rating black opal - the blacker the colour, the more valuable the stone.

Shape and Cut
This includes the overall shape and quality of cutting. Generally domed stones are more valuable than flat and regular shapes are more valuable than freeforms. Freeforms however can make very unusual and unique pieces of jewelry because of the random shape.

Size
Opals like diamonds are measured by carat weight. Quality opal can command a price per carat similar to and sometimes more than diamonds.

Imperfections
A clear opal with rich vibrant colours will be highly valued. An opal with inclusions, cracks or opaque patches will not be as valuable as a clear stone. The exception to this is boulder opal where sometimes the extraordinary patterns caused by the ironstone makes it even more rare.

Personal Preference
Unless you intend trading in opal there is no point in getting too carried away with all the technical details. Valuing opal takes into acount all of the above and more and it is rare that even two experts will completely agree.

If you see a stone or a piece of jewelry that is attractive and appeals to you then that is fine. The opal is a stone of rare and unique beauty - each person will see and appreciate it differently.