The Cleavage Planes of Gemstones

Gemstones, in addition to their visual characteristics, are defined by physical features that are crucial for jewelers, gemologists, and collectors. Among these properties, cleavage planes play an important role in how a stone can be cut, polished, or even worn as jewelry. This article explains what cleavage is, why it's important, and how it varies between different gemstones.

What is Cleavage?

Cleavage, also called cleavage planes, is the tendency of a gemstone to split along defined planes when subjected to force or shock. These planes correspond to specific directions along which the atomic bonds in the crystal structure are weaker. They are always parallel to one face of the crystal. When a stone cleaves, it splits more or less cleanly along these planes.

Here is an example to better understand what a cleavage plane is:

Imagine a paving stone made of small LEGO bricks. If you press down with a little force on a specific row, the bricks will separate cleanly along a line, creating two separate blocks. This line is similar to a cleavage plane in a gemstone.

Why is Cleavage Important?

1. Size and Polishing: Gemstones must be cut along their cleavage planes to prevent them from breaking. For example, a diamond is often cleaved before being cut, as this maximizes the use of the raw crystal.

2. Sustainability : Stones with pronounced cleavage may be more fragile and therefore less suitable for certain types of jewelry, particularly those subject to frequent shock, such as rings.

Types of Cleavage

Gems can have different types of cleavage, which vary depending on the crystal structure:

1. Perfect Cleavage:

- Description: The stone separates easily and cleanly along the cleavage planes. The surface at the break is smooth.

- Examples: Diamond has perfect cleavage, which allows it to be separated into very thin sheets. Below is a rough diamond with a typical octahedral shape (two inverted pyramids). The visible layers can be removed by striking the diamond along specific cleavage planes.

2. Distinct Cleavage:

The stone cleaves well, but the separation is not as clean as with a perfect cleavage. It is not perfectly smooth.

- Examples: Fluorite has a distinct cleavage.

3. Indistinct Cleavage:

The stone shows some degree of cleavage, but it is difficult to obtain clean, flat faces.

- Examples: Apatite has an indistinct cleavage.

4. Absent Cleavage (Fracture):

The stone has no discernible cleavage. It breaks irregularly (fracture).

- Examples: Quartz has no cleavage and fractures in a conchoidal manner (like a shell).

Cleavage in Common Gemstones

Here's how cleavage manifests in some of the most common gems:

1. Diamond :

- Cleavage: Perfect, octahedral.

Diamond cleaves easily along four main directions. This cleavage is exploited during cutting to create precise facets.

2. Topaz :

- Cleavage: Perfect, basal (along the base of the crystal).

Topaz is fragile along its basal plane, which can make it susceptible to shock.

3. Fluorite :

- Cleavage: Perfect, octahedral.

Fluorite cleaves in eight directions, making it difficult to cut.

4. Spodumene (Kunzite):

- Cleavage: Perfect, prismatic.

Kunzite has cleavage that can make it difficult to cut.

5. Calcite :

 - Cleavage: Perfect, rhombohedral.

Calcite cleaves into rhombohedral shapes, creating smooth, flat surfaces.

How Do Jewelers and Gemologists Manage Cleavage?

When working with gems with pronounced cleavage, jewelers and gemologists should take precautions:

1. Size Orientation : The stone is oriented to avoid the cleavage planes most likely to split during cutting or polishing.

2. Choice of Mounts : Jewelry settings are designed to protect stones with a pronounced cleavage from shocks and impacts.

3. Delicate Handling : Gems like diamonds are handled with care to avoid accidentally splitting them.

Conclusion

Cleavage is a key characteristic of gemstones that influences both their aesthetics and durability. Understanding this property not only allows you to better appreciate the complexity of gemstones, but also to optimize their use in jewelry. Whether you are a gemstone enthusiast or a professional, it is important to understand the specifics of cleavage to appreciate and handle these natural wonders with respect and care.