Clarity
Types of Inclusions
A diamond grading report would often include not only the overall clarity grade of the stone, but also the types and possibly locations of the inclusions present.
The clarity grade is given based not only on the appearence of the inclusions, but on their nature and whether they pose a threat to the stone's durability. There are three basic categories of inclusions: small pieces of foreign matter inside the crystal, signs of internal or external damage (usually on a microscopic level), as well as traces of natural crystal growth.
Foreign matter inside the diamond crystal is found in the form of small crystals of minerals or another tiny crystal of diamond itself, around which the larger diamond grows. They can be different in colour, transparent, or black (if the inclusion is a piece of black carbon). These are called crystal inclusions. Sometimes a diamond may have an entire cloud of multiple microscopic crystals in the same location. The individual crystals in the cloud are not distinguishable and such an inclusion is called a cloud.
Internal damage may happen when the crystal undergoes rapid changes in temperature and pressure, for example when it travels from the depth of the Earth to the surface. Such changes may change the diamond's volume which in turn may cause it to crack from the inside. Most cracks are relatively small and they are differentiated based on their direction inside the crystal. Cleaveges, fractures, tension cracks and feathers are all such inclusions. Most diamonds include at least some of these but on such a small scale that they are not visible under 10x magnification, so they do not affect the clarity grade. External damage is done to the faceted crystal during or after cutting. This most often results in blow marks, small dots on the surface of the facet or on the edges. Unless they penetrate deep into the crystal they qualify as external marks. They are usually caused by lack of care during cutting. Blow marks on the junctions of facets are often the result of wear of an old diamond. Small marks on an older diamond can often be polished off with very small weight loss to make the stone look like new.
Traces of crystal growth are the features of the rough stone that were left after cutting. They may include unpolished surfaces of the crystal, called naturals, visible lines of fusion of two diamond crystals growing together, called twinning planes, as well as cavities, which are natural indentations that are left when a crystal inclusion falls out during the cutting process.
The position of the inclusion is important for several reasons. When located close to the girdle or corners a fracture type inclusion can negatively affect the stone's durability. In addition, at certain locations close to the crown and pavilion facets the inclusion can reflect off of several facets and create an illusion that there are multiple inclusions in the same vicinity. This has a very unpleasant effect on the overall look of the diamond, as seen on the photo above, where the inclusion under the table of the stone reflects multiple times as seen from the pavilion side. On the other hand, a diamond with a very small inclusion close to the girdle can be a good choice: the inclusion can be covered with a setting and make no difference to the effective clarity of the stone, while the price of such stone would be relatively lower.
