Many first time opal buyers are not sure about what type of opal to buy. They may want to buy a semi-black opal, but may actually be looking at a black opal. Some unscrupulous dealers also try to palm off cheaper light opals as semi-black opals. So is it a sales trick or are there really semi-black opals?

Yes, there are semi-black opals. An untrained eye may not be able to tell the difference, but if you are going to buy an opal soon, it is important to at least know the difference.

Which Body Tone Range Do Semi-black Opals Fall In?

There are nine body tones in natural opals starting from N1 (the darkest body tone) to N9 (the lightest). Black opals are distributed over four body tones- N1, N2, N3 and N4. Next come the dark opals or semi-black opals. These opals are spread over two body tones- N5 and N6.

Semi-black opals have a mid-grey body tone. While black opals are mostly found in Lightening Ridge, semi-black opals are also found in Mintabie and Andamooka. The darker the body tone of the opal, higher the price per carat it commands. So black opals are generally more precious than semi-black opals and these in turn are more valuable than light opals. Note that in lighter opals (N7, N8, N9), transparency is more important.

Other Properties of Semi-black Opals

These opals can either be translucent or semi-translucent. Translucency is the property of a substance that allows light to pass through. But once it enters the opal, the light is diffused in such a way, that no proper image is formed on the other side. Black opals are also more opaque (see through ability) than semi-black opals, so this is another major criteria that separates black opals from semi-black opals.

Shapes in which Semi-black Opals are Available

Opals are opaque stones, so in contrast to transparent precious stones, they are usually cut in a cabochon shape. They are polished and shaped in such a way that the top has a convex shape, while the bottom of the stone is flat. Transparent stones, such as diamonds and rubies are faceted.

So, you have semi-black cabochon opals in different shapes such as neck tie, trapezoid, freeform, triangle and oval shape. Like all precious opals, semi-black opals will also have flashes of color or 'fire'. It can either be a multi-color flash, a green flash or a blue-green flash.