Like colors, gemstone transparency – whether a gem is opaque, translucent, or transparent – is a result of light interacting with the gem. Although gemstone transparency is not a precise basis for identifying gem species, the presence and degrees of translucence and transparency in particular gems can have tremendous subjective appeal to collectors and can also affect a gem’s value.
At the atomic level, gemstones absorb, transmit, and reflect different wavelengths of light. The wavelengths of visible light that a particular gemstone transmits are what we see with the naked eye as the gem’s color. Gemstone transparency is a way of describing how much light, if any, a gem can transmit and how scattered that light is when it exits the gem.
Opacity, Translucence, and Transparency
The more wavelengths of light a gemstone absorbs or reflects, the more opaque it is. (Both a solid black object and a mirror, for example, are considered to be opaque). The more light that is absorbed or reflected means there is less light to transmit through the gem.
A translucent gem can transmit enough wavelengths of light that we can see light shining through it. However, if the wavelengths of light that escape the gem are highly scattered, due to the gem’s refractive index (RI) or other physical properties, light is all we can see through the gem. We can’t see images of what is beyond it.
A transparent gem is translucent and transmits light with little scattering. This allows us to see images as well as light through a transparent gem.
Can You Measure Gemstone Transparency?
Although it is possible to measure scientifically the opacity of a material, from perfect transparency to perfect opacity (in units of “opacity,” no less), the commonplace meanings of opacity, translucence, and transparency are what are normally used in gemstone descriptions by gemologists and jewelers. Since there can be a wide range of transparency even within a single gemstone species, determining whether a gemstone is opaque, translucent, or transparent doesn’t necessarily help identify it. Precise measurements, in most cases, aren’t necessary. Gemstone transparency descriptions such as “near opaque,” “semi-translucent, and “almost transparent” can convey well enough the optical effects of interest to most people.
Nevertheless, there is an important scientific distinction to make when discussing gemstone transparency. Since we can only see white light wavelengths with the naked eye, our observations of opacity, translucence, and transparency take for granted that we are referring to these conditions in the visible spectrum of light. However, there are wavelengths of light beyond visible light, such as ultraviolet (UV) light. Some gemstones, such as diamonds, are transparent into the UV spectrum. Type I diamonds, the most common type of diamonds, contain nitrogen and are transparent down to UV wavelengths of 300 nm. Type II diamonds contain no nitrogen and are transparent down to UV wavelengths of 250 nm. In white light these types of diamonds would have no noticeable difference in transparency. A spectrometer would be needed to make this distinction.