Gems sometimes have one or more cleavage planes. It is a plane along which the gem can split into two parts more or less cleanly.

The gem cutter must take this kind of detail into account to avoid problems during and after cutting. We will therefore take a simple example to explain the method to use: topaz.

Topaz has a perfect basal cleavage, which means it cleaves perpendicular to the c-axis: the growth or elongation axis of the crystal.

Often, topaz crystals are elongated and they encourage the cutter to place the table parallel to the c-axis to preserve weight.

In practice, to prevent the cleavage from aligning with the foliation, the table is slightly tilted.

The method therefore consists of orienting the table at least 7° away from the cleavage plane.

Explanatory diagram :

If the table were placed perpendicular to the c-axis, the same method would be used to prevent the table itself from aligning with the cleavage plane.

Risk of cutting along the cleavage plane?

The facet to be cut could chip instead of becoming perfectly flat.

A facet could end up in the cleavage plane, making polishing difficult or even impossible.

During setting, the jeweler will apply downward pressure, and it is better to avoid having it along the same axis as the cleavage, as this would pose a risk to the stone.

A stone with poorly oriented cleavage, worn in jewelry, would be more likely to break upon impact.

Now that we have presented the case of a perfect crystal, you might wonder how to reveal the cleavage plane if the gem appears as a rolled pebble, for example?

It is not obvious, but there may be clues to guide us. The crystal could contain traces of cleavage: cleavage propagates in a straight line, unlike fractures which take random directions, and it is difficult to confuse them.

In the example of rolled topazes, they might show two more or less flat and parallel faces, which could correspond to the cleavage direction. Still, be careful not to confuse them with two crystal faces.

If there is no way to identify the cleavage, the best approach is to at least test the future table to see if it polishes easily. With a bit of luck, the other facets will also be outside the cleavage.

For simplicity and accessibility to the widest audience, I will not go into overly technical details.

However, there are gems that have multiple cleavage directions: sphalerite, spodumene, etc.

The method to use remains identical to the one mentioned above, taking into account that the rule applies in several directions of the crystal. Therefore, the table of the stone must be oriented so that the facets (including the table) are a few degrees away from the cleavage plane.

As with soft gems, I do not recommend this type of material for beginners. Cutting could quickly become frustrating due to the difficulty of the work ahead.

In any case, I wish you good orientation and, at the same time, a successful cut.