A knot crystal is the embedded diamond inclusion that gives rise to a knot as catalogued in the GIA inclusion nomenclature. It is, in mineralogical terms, a diamond crystal that grew within or alongside the host diamond crystal during formation, sharing the same carbon mineralogy but differing in crystallographic orientation and, often, in trace nitrogen or boron content. When the host crystal is cut and polished into a faceted stone and the embedded crystal lies at or near a final polished surface, the inclusion is exposed and is reported on the diamond grading plot as a knot. For a treatment of the surface-feature appearance and grading effect, see the related entry on knot.

Formation

Knot crystals are syngenetic inclusions, that is, formed simultaneously with the host diamond in the deep mantle, generally in the lower lithosphere or transition zone. Their crystallographic orientation differs from the host, which is the proximate cause of the differential polishing rate that gives the knot its visible boundary in the finished stone. Trace-element analysis by SIMS and EPMA on knot crystals has been used in research at the GIA, the Diamond Trading Company laboratory and the academic community to characterise the conditions of diamond formation, since the relationship between the knot and the host can preserve information about the timing of crystallisation and the chemistry of the surrounding mantle fluids.

Appearance and effect

Within the rough crystal, a knot crystal is invisible to ordinary inspection unless it is exposed by cleavage or sawing. Once a face of the host has been polished through to the knot crystal, its differential hardness on different crystallographic axes and its differential reaction to the polishing lap produce the characteristic outlined patch on the finished facet. In the rare case where the knot crystal contains a trace of brown, pink or yellow colour, the inclusion appears as a faint coloured spot rather than as a colourless boundary, which can occasionally raise small questions of identification under ten-power loupe.

Distinction from other features

A knot crystal is to be distinguished from a non-diamond crystal inclusion, which is reported as crystal rather than knot on the GIA grading plot. Non-diamond crystals include garnet, olivine, chromite, sulphide and graphite inclusions, all of which appear in diamond and which present differently under inspection. Knot crystals are also distinct from twinning planes and from grain boundaries within the host, which are reported under their own headings on the grading nomenclature. Identification of the inclusion type relies on inspection of the bounding line, the colour and translucency of the inclusion, and, in difficult cases, on Raman spectroscopy in the laboratory.