What differentiates one stone from another? What makes a sapphire a sapphire and a tourmaline a tourmaline? What makes them different? These are simple yet extremely important questions, and they are the basis of gemology. At first glance, we often refer to the color of a stone. This is a good indicator to get a first idea of the variety of a stone (a blue stone cannot be an emerald for example). But this is not enough: this same blue stone could be a sapphire, a zircon, a tanzanite, an aquamarine, a topaz, an apatite... To be able to determine precisely which stone it is, it will be necessary to look more closely at its characteristics.
A gemstone is a mineral or organic material created in nature through natural processes. Each stone is characterized by a set of physical, chemical and optical properties.
The physical properties concern the internal structure of the stones: each stone is an assembly of atoms and molecules arranged according to a well-defined structure - the crystal structure. From this arrangement of atoms comes the hardness, for example, evaluated according to the Mohs scale. A mineral with a hardness of 8 (topaz...) for example will scratch a mineral with a lower hardness (like emerald, with a hardness of 7.5). The chemical properties concern the elements that make up the stone, and the optical properties the appearance of the stone.
Some of these properties are very important to gemologists: they are the ones that will be studied by gemological instruments, and that will allow to certify that a ruby is a ruby and not a garnet.
- Refractive index and birefringence: when a ray of light passes through a stone at a certain angle, it will be deviated through the stone. The refractive index measures this deviation. In addition, a ray of light will emerge as a single ray or as two rays, separated by a certain angle. The stone then has two refractive indices, and the difference between them is the birefringence. To measure them, we use a refractometer.
- Density: some stones are heavier than others - this is what density represents. To measure it, we use a hydrostatic balance.
- Pleochroism: this property concerns all stones that have two indices of refraction. For these stones, depending on the direction in which one looks at it, its color can change. Some stones have a stronger pleochroism than others: an unheated tanzanite, for example, will be in a blue direction, in another purple and in a third will have a brownish hue. A dichroscope is used to measure this.
- Absorption: Each colored stone absorbs a portion of natural white light. Each variety of stone has a unique absorption, and it is possible to visualize it with a spectroscope. If you are interested, you can learn more by reading our article on the cause of gemstone color.
It is often the combination of these different characteristics that can attest to the variety of a stone. For example, to certify that a stone is a ruby, we look at its refractive index, its birefringence and its absorption spectrum. Color and density are also very good indicators.
The world of gemstones is extremely vast, and distinguishing one variety of stone from another with the naked eye is difficult. You can get an idea, but a more precise study of the stone is often necessary, and this is where gemological instruments come into play. To help you see more clearly and know how to identify the most common stones, we have prepared a small summary table. If you have any questions, don't hesitate to contact us, we'll be happy to discuss them!