Feldspar and Color-zoned Bands

Gemstone quality feldspar is neat! Feldspar is found in many forms on Earth and in space. Commonly traded gemstone feldspars — Moonstone, Sunstone, and Labradorite — owe their popularity to unique optical effects. Highlighting, understanding, and exploring how these effects present themselves in feldspar, and how they correspond with fracture and cleavage, is worth a few words.

As feldspar forms in cooling magma, it usually bonds in layers. Alternation between hotter and cooler temperatures causes these layers to have slight variation in molecular structure. These varied layers help create a lot of the light optics found in feldspar, such as labradorescence and adularescence. These glowing effects can be seen below.

Layers of Crystals: Feldspar Cleavage

Unfortunately, these layers are bonded weakly to one another, and this weak bond, or cleavage, is what keeps a lot of feldspar gemstones away from the jeweler, cutter, and wearer. Cleavage in gemstones is where the material wants to naturally cleave along these weakened bonds; it’s a lot like wood grain but on a molecular level. Feldspar has a second direction of cleavage that is always 90° with the first. Finding one plane, can help you determine where the second will be.

Faceting or cutting feldspar parallel with either cleavage plane will cause a layer to cleave off, which leaves an un-polished surface. If it cleaves in both directions simultaneously it will leave a fractured surface.

Sunstone with a perfect split on a cleavage plane.

Besides issues of polishing, the cleavage also leaves the stone vulnerable to splitting into larger pieces. Usually this happens when the stone takes on a decent amount of force, such as hitting the ground or dop failure. Though this is all very daunting, the stone structure is strong enough that this cleavage doesn’t usually present itself until the pre-polish phase (3k–14k grit).

How Does a Lapidary Avoid Cleavage Planes?

Coordinating the design to avoid the cleavage planes landing on one of the faces of the stone helps save a bit of hair. It requires careful observation of a selected feldspar to discern the two planes, and will be useful toward understanding how best to approach cutting and setting any feldspar.

Finding these cleavage planes tends to be a difficult task when dealing with translucent feldspar material. In this case, the cleavage planes can be found with work on a wheel/tumbler. Eventually with this work you will see the material cleave, and take note. However, if a piece of material has observable optical effects, its planes of cleavage are revealed.

Find out how the angle of the optical effect is coordinated in the stone so the only light source is parallel with the eye. In other words, start with a light directly above your head and the stone. Your eyesight and the light need to be perpendicular to the stone so you can determine the angles. Where things get difficult, the second cleavage plane is 90 degrees to the first, but this can be in two different directions within the stone; north-south to you or east-west to the plane. There’s not really much, as far as my experience goes, in figuring out how to best find these secondary angles without the optical effects highlighting the planes.

Here is a 15.8g piece of orthoclase feldspar. This feldspar has pink and clear banded color zoning. Pink in orthoclase is due to iron getting in the mix. These stacked planes do let me know where the cleavage planes lie.

The general consensus among my peers is that a plane should be 15% +/- off the table of the stone when faceting feldspars.

The next time you find a really wondrous color-zoned piece of feldspar, with optical effects that bring out distinct angles and planes, let me know how your cutting and design went!

Further Information