The use of lasers in treating cut diamond has. been has been commercial practice since around 1970.
Its aim is to improve the optical appearance of diamonds which are difficult to sell because of their dark inclusions (magnetic pyrites and magnetite), by burning out or bleaching the. inclusions or by chemically dissolving them with an etching fluid (sulphuric acid and saltpeter).
With the use of a special drilling system, tiny drill-holes as fine as a hair can be produced by means of a laser beam. The laser beam is focused in the optical axis of the microscope on the surface of the diamond. The focus of the laser beam is marked by cross hairs and is on the same plane as the focus of the microscope. With the use of two objective lenses total magnification is optionally 45- and 120-fold. Combining laser and microscope optical systems and a microscope attached to one side permits constant examination and control of the drilling operation. Adjustment of the diamond requires a considerable amount of time. Depending on the position of the inclusion, it can take up to two hours.
The diamond is held in a clamp which is attached to a ball and socket, so that adjustment in the desired drilling direction is made very easy. The ball and socket can be locked in any position and is fitted with a self-centering base. The holder can therefore be taken out and put back during the drilling without change to the drilling geometry. The holder is attached to a support which can be shifted 40 mm in X Y and Z directions respectively by means of a micrometer. The distance of an inclusion from the surface of the diamond can be easily ascertained with a depth gauge. The diamond is illuminated from above and also from the side. Diamond drilling is possible to a depth of 1.6 mm with a diameter of only 20 to 60 microns maximum. Twenty microns are 2/100 mm. This is equivalent to the breadth of a human hair. For drilling to a depth of 1.5 to 2 mm maximum, approximately 30 to 45 minutes are needed. The depth and diameter of the drill-hole are determined by a diaphragm in the laser head which can be adjusted with a micrometer screw.
In the case of an inclusion which appears dark because
of total reflection (air is the boundary surface of a diamond), air
penetrating through the laser drill-hole can alter its appearance: the
crack becomes light.
Another possibility is for a liquid to be injected under pressure (in a
vacuum) into the drilling channel. Depending on the properties and
chemical composition of the inclusion, the liquid will bleach, etch or
dissolve it through decomposition.
Of late laser-drilled channels are filled with a highly refractive wax or synthetic resin and sealed on the surface. The drill-holes become slightly less clearly visible as a result and provide protection against the penetration of dust and dirt. There is no problem in discerning the drilling channels with a 10 x magnification, even if they are sealed. The drilling channels appear as funnel-shaped cavities. If the laser drill-holes pass through areas of strong internal tensions, whole tension-crack and cleavage-crack whiskers frequently form around the channel (Figs. 39 and 40). A small "crater" (hole) is seen on the stone surface, and drill-holes can be clearly discerned, especially in reflected light (Fig42). The circular depressions can be clearly felt with the tip of a needle.
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| Fig 39 Laser drill hole showing cleavage cracks perpendicular to it and a funnel-like opening (20 x) | Fig 39a As Fig 39 but 40 x |
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| Fig 40 Three laser drill holes showing
definite cleavage cracks (10x) |
Fig 40a As Fig 40. but 30 x |
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| Fig 41 Three laser drill holes. seen from the side (30 x) | Fig 42 As Fig 41. but seen from above perpendicular to the openings of drill channels |
Grading stones like these sometimes poses problems, as
while the inclusions have a better appearance, new and especially
artificially produced internal defects are added by the drilling
channels.
Classification in a particular clarity grade depends on appearance, with
account being taken of any drill-holes present. The whole idea behind
laser treatment is not to improve a clarity grade, but to make the
diamond more saleable by lighting the inclusions.
Laser drill-holes have to mentioned in any event in certificates, invoices, offers etc., as they represent an artificial intervention.
| Clarity Grading >>Description of inclusions | Filled Diamonds |