General
The process of the growth of a diamond in the interior of the earth did not occur evenly, but in several phases. In these various phases, the conditions pressure, temperature and cooling - did not always remain constant. In homogeneities occurred, which are now found as internal features, the so-called inclusions. They are present in the shape of clouds or turbidity, they form fracture, cleavage or tension cracks, or they constitute enclosed minerals which were present during the growth process of the diamond. All such inclusions often serve to-day as diagnostic identification features and are an indication of the natural origin of the stone.
For a long time the nature of these inclusions was not investigated, all dark inclusions were termed "carbon", the light ones "ice" or "snow". Only with the increasing interest in the diversity of the internal features it was seen that the dark appearance of many inclusions was based on an optical illusion; because of the lower refractive index of the enclosed mineral, total reflection occurs at the boundary between the diamond and the inclusion, so that the latter appears black from certain directions of viewing. The same phenomenon can be observed in the case of cracks.
Informative research work by GUBEUN, EPPLER and other authors led to the realization that inclusions in diamonds were formed in three phases of growth. One differentiates between inclusions which were already present before the crystallization of the diamond and were enclosed in it (pre-existent inclusions), others which were formed at the same time as the diamond (syngenetic inclusions) and a third type, which developed subsequently (epigamic inclusions). The last type, for instance includes cracks which resulted because of stress after temperature or pressure variations or because of irregular cooling.
About twenty five different minerals are known as inclusions in diamonds, the most common being reddish garnet, brown Spinel, green enstatite and diopside as well as dark brown to black ilmenite and magnetite. Also dark graphite can be found as inclusion in a diamond crystal as a product of metamorphosis.
Much more information on the nature of inclusions in diamonds can be found in the book "Photo atlas of Inclusions in gemstones" by Gubelin / Koivula.
- General
- Origin of clarity grades and determination of nomenclature
- Clarity Grading
- Description of inclusions (Cleavage, fracture and tension cracks, feathers, crystal inclusions, structural distortions, twinning lines and planes, growth lines and planes)
- Laser-drilled diamonds
- Filled diamonds
- Conditions of Clarity Grading
- Definition of Clarity Grades
- Illustrations
- Aids to Clarity
Grading
- Loupe
- Models of Microscopes
- Instructions for Practical Clarity Grading
| Summary table | Origin of clarity grades and determination of nomenclature |