Light and Colours
Before completely immersing ourselves in the study of gemstones, we must not overlook the last property that is essential in determining the beauty of gems: light.
What is the relationship between the light and the colour of gems?
These two properties are tightly linked, because the primary determinant of the colour of a gemstone is the selective absorption that is caused by the crystal structure and the chemical composition of each mineral. More generically, we can say that the beam of white light that enters into all objects (not only minerals) is composed of electromagnetic waves of all the spectrum colours (red, orange, yellow, green, blue, violet). Every object, when struck by light, absorbs certain colours and releases others. The colour we see is the one that has been released to our vision. It is called the base colour (see design on the right). Are light sources always the same? No. Possible sources of light include sunlight and artificial light. Artificial light is similar to sunlight, though it does not have the same intensity or wavelength, which causes us to perceive it in a different manner. Let’s look at how.
Sunlight: though not always available, it is the ideal light for many gemstones, especially red, orange and yellow ones.
Incandescent light: similar to sunlight, it is yellow, hot and extremely intense. It is suitable for all gemstones that are yellow, red and orange in colour.
Fluorescent light: a cold, clear light that does not transmit shades. It is generally suitable for all gemstones that are violet, blue and green in colour.
Up to this point, hopefully we have not left anything out.
In reality, we have not said much about colours, and a careful reader may reprimand us for this. However, the complexity and importance of the subject do not allow us to fully cover it.
We will limit ourselves to say that for the entire gemmological world, colour is the most fundamental factor used in evaluating and recognising a gemstone! In fact, the characteristics considered when determining the value of a gemstone are the famous 4 Cs, which are best known for describing diamonds: Colour, Clarity, Cut and Carat. And if colour is so important for the king of all gems, think of what it means for our “coloured gemstones!” It is the intensity of a gemstone’s colour, called its “saturation” by the experts, that gives a stone its value. At times the intense colour of a gemstone can even make up for internal inclusions!
We have seen how the colour of individual gemstones is determined, so now let’s look at what this colour is composed of. Colour is a tremendously important factor for gem lovers, and is the most useful guide for the identification of gemstones with the naked eye. The optical aspects of colours differ from one another depending on the following three attributes:tint, tone, saturation.
Tint is the chromatic impression that is perceived with a glance. In other words, it lets us describe a colour as green, blue, etc. Just think that the human eye is capable of distinguishing approximately 150 pure tints. Tone is the relative brightness of a tint and is based on a scale that goes from light to dark. Saturation is intended as the concentration or pureness of a tint. It is the vivacity of the colour.