Topic 2: The Structure Of Diamonds And The Implications For Polishing.

A. Four points in sawn stones are identified from drawings.

Four points in sawn stones are identified from drawings

B. The polishing grains in four point sawn stones are identified in terms of the crystallography of diamonds

The polishing grains in four point sawn stones are identified in terms of the crystallography of diamonds.
The polishing grains in four point sawn stones are identified in terms of the crystallography of diamonds

C. Problems that can be encountered when polishing four point sawn stones

The types of problems:

Glitzes

Naat

Cross-grain

Holes

Flaws

Impurities

Naturals

Every one of the above is challenging the worker/learner to use the knowledge of his trainer extensively. This knowledge can only be obtained through years of experience and gradual adaptation and keeping abreast with all new scientific and technological discoveries.

Stress:

Caused by the presence of high internal strain on the diamond.  Strain is also a symptom of stress caused by minute inclusions or “knots” which polishers try to avoid.

Regions of strain can be identified from optical birefringence.  Patterns are determined by using a stress lamp also known as a “Polariscope”.

Diamond parcels containing stones with stress will be clearly marked and the polisher must use extreme care when polishing top stars.

Inclusions – Flaws – Impurities

Inclusions in diamonds are regarded as undesirable defects, while facts can introduce multiple reflections of inclusions in the view facing the rib.

Such inclusions may harm the quality of the materials and its ability to withstand heat and pressure.

Pinpoints

Small flaws, which are seen when, viewed through a 10 x loupe.

Carbon Spots

All small black or dark spots are known as “carbon spots”

Clouds

Comprised of a large number of minute crystals resembling a cloud present in the stone.

Knot

Also known as “naat” and happens when the included crystal is exposed at the surface during polishing.

Cleavages

Cracks (gletzes) in the diamond which coincide with the diamond’s cleaving direction.

Fractures (cracks)

When the gletz in the stone does not follow the cleaving direction.           Sometimes the crack resembles a small feather, therefore, not surprisingly, it is then called a “feather”

Graining

Cross-grain occurred when the crystal structure of the diamond is not parallel.

Holes

A small hole or empty cavity in the surface of the stone may have resulted from an included crystal that was torn free from the facetted diamond during polishing.

Naturals

A natural within the girdle area, must not penetrate the body of the stone, cannot be viewed from the top (crown) and does not affect the symmetry of the girdle.

Heat

Polishing by applying rather high pressure on the stone, which has been placed on the scaife in a “worn down” polishing circle, or against the grain. As is in the case of all materials, extreme temperatures open the pores, giving the stone a greyish burned surface, which can only be removed by re-polishing.

Naat

A “naat” is present when the crystals in the diamond are not in the same direction or through twisting and squeezing together with several already formed diamonds. A naat stone can be identified by a line or lines, which may have an irregular pattern and a clearly visible division between two or more crystallizations systems. This is something, which must be taken into account by the polisher.

Methods of dealing with problems encountered: –

  • Do not use excessive pressure if the correct polishing grain is not yet known – the stone can crack on the naat
  • Polishing grains on naat stones deviate from normal crystallization, so the original polishing grain is also influenced

Experience has taught us that good results can be obtained by polishing across the naat or parallel to it.

D. Faceting hints and techniques for bottom blocking four point and the consequences of using inappropriate polishing techniques.

Bottom Blocking (Corners)

To block the four bottom corners, the longest edge for a octahedron is generally selected.  For a dodecahedron, the face with the deepest natural is selected.

The rough stone is twisted to the right (bottom corner +/- 3° to 5°) and the dop is turned to “In-de-Tang”.  After testing, the first corner is polished at 42° angle.  The polisher takes the girdle into account leaving enough rondist (girdle) to polish the crown.  (See Fig 09)

The second bottom corner is then blocked by skipping two clicks on the dop (when working with an 8 click dop as there are also 16 clicks dops) After polishing the four bottom corners equally deep into the girdle on four sides, the culet should end at a point in the centre of the stone.  This finishes the bottom blocking.

The advantage of this is that after having polished the second facet, opposite the first, the final position of the culet can be checked as well as whether the girdle of the stone is thick enough.  This is especially important when working larger rough stones.

Bottom blocking pavilions

Blocking the intermediate pavilion facets, the dop is turned to the “Tege-Uit” position. This pavilion facet – the fifth is obtained by one click on the dop, the sixth and the next to pavilion facets by moving two clicks each time (with an 8 click dop). The blocking of the bottom side is finished when the culet is straight in the middle.