Gemstone Properties: Diaphaneity of Gems & Minerals
The Transparency & Opacity of Gemstones
The transparency or opacity on a mineral is referred to as its "diaphaneity," which describes the quantity of light that is able to travel through a material. The quantifying of this physical attribute can be subjective as there are several factors which can inhibit light transfer, none of which are directly related to the chemical/physical composition of the mineral itself. These factors include:
- Thickness of the specimen
- Presence/absence of inclusions
- Alteration, tarnishing or weathering
It goes without saying that the thicker/larger a translucent specimen is, the more opaque it will seem. For a mineral to be considered as "transparent" it must be totally clear (not the same as colorless), with no interference in the transmission of light.
Very opaque minerals such as lapis lazuli, malachite or pyrite (above, right) rarely occur in fully transparent form, but relatively opaque minerals such as Jade due exhibit translucency. Minerals such as >agate, carnelian and chalcedony all appear as translucent when the specimen is cut thin enough to transmit light.
There are several standardized terms in the gem trade to describe a mineral's opacity, although there is not a standardization for specimen thickness when assessing their diaphaneity. These basic categories are:
- Transparent (Tp)
- Semitransparent (S-Tp)
- Translucent (Tl)
- Semi-Translucent (S-Tl):
- Opaque (Op)
Within these five basic categories there are several sub-categories that provide a more exacting and nuanced description. They are: semi-transparent, sub-transparent, semi-translucent, sub-translucent, semi-opaque and sub-opaque.
The presence of inclusions can play a big part in the diaphaneity of a material. Varieties of corundum (ruby, sapphire) can be heavily included with rutile which can render them nearly opaque. The beryl sample above (center/right) is an example of how alteration and weathering can create, or exaggerate opacity in an otherwise translucent specimen.
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