Characteristics

Hardness

The hardness of a mineral is usually the scratch hardness, meaning the resistance to scratches. It is measured with the Mohs Scale, which was introduced by Friedrich Mohs in 1812. The hardness is measured with values from 1 to 10. The harder material scratches the softer material.
The scale is not linear, for example Corundum is twice as hard as Topaz, but Diamond is almost four times as hard as Corundum.
You can see this below, when you compare Mohs hardness to the absolute hardness measured with a Turner sclerometer invented by Thomas Turner in 1896.

It is worth mentioning that testing the authenticity of a gemstone by scratching it is probably not the best idea, as you do not want scratches on the surface of your favorite gemstone. And you might even break it depending on the cleavage of the stone ( see below ).

It is better to test authenticity with e.g. the absorption spectrum ( see below ).

You should also consider that normal dust contains small sand particles. Sand is quartz, so that wiping of the dust can leave small scratches on the surface of a softer gemstone, which reduces the shine with time. Gemstones with a hardness of 6 and more are robust enough for daily use.

Cleavage

A mineral has a crystal structure, this is how the atoms are arranged in the material. The atoms are held together by cohesive forces. These forces vary within the structure, which means that there are directions in the crystal structure where the crystal cracks easier than in other directions. How strong the atoms hold together is given by the cleavage and there are several grades of cleavage:
- Perfect
- Good
- Fair
- Poor
- None

For your orientation, diamand cleavage is perfect. This fact is actually used to split raw diamonds into smaller pieces. But it also means, that you better not drop your diamond juwelry on a tiled floor.

On the other hand the cleavage of Quartz is none, which makes the Quartz a perfect gemstone for daily use. It is not as hard as a diamond, but looks very similar, when it is perfectly clear. Not to mention the price difference...

Cleavage also needs to be concidered for the cut of a gemstone. Gemstones with good cleavage that have a cut with pointed edges, should have a setting, that protects the pointed edges.

Weight

Traditionally the weight of cut gemstones is measured in carat (abbreviation ct.) . Bevor 1907 the carat was not standardized and the weight varied between 188mg and 213mg.
Today the metric carat is used and 1 carat equals 200mg=0.2g (gram).

The weight of tumbled stones and Cabochons is often given only in gram, not in carat.

Colour

The colour is very important for a gemstone, as it is the colour, transparancy and shine, that makes a gemstone so beautiful and attractive.

The colour of a gemstone is determined by the wavelength of the light that is absorbed inside the stone. Light of all visible wavelengths together makes the stone appear white (clear). If for example blue light is absorbed by the stone, but red and green light is emitted, the stone appears yellow (additive mixture of red and green). This is a simplified explanation of the colour appearance of gemstones. It also explaines, why gemstones may have different colours in natural and artificial light. In the example above, if green light is missing in the artifical light, the stone would appears red.

The colour of a gemstone is mainly determined by metal atoms, like chrome, iron, copper etc, that absorb light of certain wavelengths. The quantity of these atoms vary even in the same type of gemstone, depending on where they come from. This is the reason why most types of gemstones can vary in colours and the colour is not enough to determine the type of stone. A Garnet is not always red.

The amound of light of all wavelengths absorbed by the stone determines the transparancy of the stone. If next to no light of all wavelengths is absorbed, the gemstone is clear, more absorbed light of all wavelengths makes the stone translucent and when all light is absorbed, the stone is opaque.

Absorption spectrum

As mentioned above the colour of agemstones is determined by absorption of light of certain wavelength, mainly through metal elements in the stone. But for the human eye red is basically red, so that it is difficult to tell the difference between e.g. red Pyrope and red glass.

But if you look at the light spectrum that is emitted by the gemstone, you can see black ribbons where the light of this wavelength is missing in the spectrum. This is almost like the fingerprint of the gemstone and helps to identify the type of gemstone. This absorption spectrum can be seen with a spectroscope. As light needs to pass through the gemstone, it works best with coloured transparent gemstones.

With this method it is also possible to identify imitations and synthetic stones and it works even with set stones without any damage to the gemstone.

Transparancy

If the light passes through the gemstone with no or little absorption, the gemstone is transparent. If more light is absorbed, the gemstone is translucent and if all light is absorbed or internaly reflected, the gemstone is opaque.

Most faceted cuts work best with transparent stones and transparent stones are the most valued. But also intransparent stones are very beautiful, like black tourmaline, lapis lazuli, turquoise and others. Intransparent stones look very beautiful when cut and polished to cabochons or balls for example for a necklace.

Shine

The shine of a gemstones is caused by reflection of light on the surface. Additionally certain facetted cuts, like the brilliant cut, reflect light, that enters the stone, internally through total reflection on the facettes (like a mirror), so that this light is emitted again through the surface. This effect adds to the natural shine and makes the gemstone brighter, more brilliant so to say. Of course this works best with very transarent gemstones with very little internal impurities, like inclusions, cracks or bubbles.

Some gemstones also show special light effects on the surface. One of the most famous is chatoyance, also named cat`s eye effect. The gemstone Tiger eye is well known for this effect and it makes it such a fascinating gemstone.