Let's go back to beauty. Some of the artificially colored stones imitate other stones. But there are also such, like diamond or agate, which remain themselves after coloring. They are only more beautiful. You can also make stones more beautiful by other methods – by polishing them, increasing the smoothness of their surfaces and the ability to reflect light, and by changing their shape.
The ability to reflect light depends, among other things, on. from the smoothness of the surface. The ancients noticed this, and they polished the surfaces of the crystals they found for a long time. They also noticed, that polished stones have a more intense color and greater brilliance. But the price of each stone has long depended on its size. The cut used by the ancients was therefore sparing, superficial, such, that the loss of limescale is as low as possible. Then, observing the natural shapes of the crystals, they found, that the brilliance of a stone depends not only on smoothness, but also on shape. Primitive tools and methods, however, limited machining to simple and oval shapes. A cabochon cut was created, convex and rounded, used to this day, but only - for opaque stones, jak lazuryt, our Polish chrysoprase, and especially opal, cat's and tiger's eye and others, which iridescent, interfere with or show similar lighting effects.
Hard and expensive diamonds and rubies were still polished, or rather, it was cleaned only on the surface, removing the outer layer of impurities and smoothing the natural crystal surfaces. It was not until the 15th century that the size of the stone was abandoned, the polishing of diamonds and other stones in the shape of a polygon began, to after 200 years of trials, often leading to the destruction of the diamond, to develop the so-called. brilliant cut, and after the following years, at the beginning of the 20th century, when the laws of the physics of light were learned, develop a mathematical principle of grinding gems. A rule that makes it easier to obtain gems with the highest brilliance and the most beautiful play of light.
Contemporary "ordinary” diamonds are cut 58 planes, i.e.. facets. In special cases, it is ground up to 104 płaszczyzny, even losing 60% stone mass. Is it worth it?
To, what the masters of the Middle Ages and the masters of rebirth did experimentally, we can justify with the laws of physics. The lower part of the figure opposite shows the diamond in cross-section, with two rays falling on its surface. Part of each ray goes into the diamond, it reflects off one or more of the interior surfaces and returns through the top surfaces to the outside. The shape of the diamond was calculated this way, so that all or most of the light, which will enter its interior, it went out through its upper part towards the viewer.
Two basic cuts of stones: cabochon and diamond.
The same phenomenon is responsible for the good visibility of the reflectors, so the diamond "shines."” own light, reflected from its internal surfaces. The angle of the diamond base flare is selected individually for each stone, but the greater the refractive index characteristic of the stone, the more light comes out of it towards us. The refractive index of water is 1,33, for glass 1,5, quartz only 1,55, for crystal glass and for corundum – 1,77, and for a diamond as much as 2,42, so diamond is the best. Among gemstones, only zirconium has a refractive index of 2.02 and can therefore be an imitation of a diamond.
Simultaneously with the refraction of light, it is split into colored spectrum components. This is shown on the example of a radius drawn with a continuous line. We remember, that each of the colored rays refracts at a different angle depending on the wavelength. The difference between the refractive indices of the extreme components of the spectrum, hence red and purple light, is called dispersion. The greater the dispersion value, the farther apart the rays of red and violet light emerge from the interior of the stone. The more colorful the gem becomes, then we speak of "fire” precious stone. Ordinary glass shows no dispersion, does not diffuse light. The dispersion value for quartz is only 0,013, for the ruby 0,018, for one of the varieties of grenade-pyrope 0,027, and for a diamond 0,044. That is why the diamond is said to be, that he has a very strong fire. But this lighting effect also depends on the shape of the diamond. All the colors of the light must be fully reflected inside the gem and output through the top surface. Otherwise it can happen, that a colorless clear diamond will give the impression of e.g.. red. The dispersion of zirconium is as high as 0,038, zircon is also like a diamond in fire. Is a diamond and in this field is the best? Contrary to expectation, no. As many as four minerals are characterized by a higher dispersion value. The highest sphalerite 0,156, almost four times the size of a diamond. Unfortunately, sphalerite is a very soft mineral and is not suitable as a material for making gems.
Of course, not all stones are polished into a diamond shape. In part, it depends on the optical properties of the mineral, and in part from our fantasy. Most often, apart from brilliant, we use a step cut, also called emerald, koptowy, star-shaped or scissor-type and the so-called. fancy-heart cuts, water drops, etc., etc.. All for this, to bring out as much beauty as possible from the stone.