Any gemstone having the potential for an optical phenomenon will be difficult to cut, and stones having Adularescence – or Schiller – are particularly difficult to orient for cutting. Moonstone is named for this effect, which gives the impression of silvery lunar light.
Schiller is often described as a blue or milky luster or glow that originates below the gemstone’s surface and appears to float across the stone as it is moved. Adularescence can be found in many gemstones of the Feldspar group, including Moonstone, Sunstone, Amazonite, Labradorite and Spectrolite.
Moonstone can be Orthoclase Feldspar or Albite Plagioclase Feldspar and is relatively tough, ranking at about 6 on the Mohs Scale of Hardness. Typically white, Moonstone can also be found in a variety of colors and can also be opaque or translucent. It is a readily available gem material, and quite popular for jewelry.
Properly orienting Moonstone before you make the first cut will optimize the stones adularescence and showcase the blue silver flash that gives the gem its name. Here’s an easy way to help you position a piece of Moonstone rough so you can create a pair of similar size matched cabochons.
Step One: Place the Moonstone rough on a table with a small, bright halogen lamp suspended directly above the stone.
Step Two: Position your head so you can look straight down on the moonstone without casting a shadow on the stone.
Step Three: Begin to rotate the stone in various directions until you notice a bright silvery yellow color on the stone’s surface. Mark that exact spot with a graphite pencil.
Step Four: Turn the Moonstone rough over completely, and repeat the previous step until you locate a second bright silvery yellow flash on the second side of the rough. Mark the second spot with the pencil too.
Step Five: Do not move the moonstone. Using the pencil, draw a line around the stone’s equator. The marked dots should be centered at the top and bottom, like north and south poles.
Step Six: Use a lapidary saw to cut parallel to the marked line. Once sawn, the cut surfaces should be positioned as the base for each cabochon. Use the pencil to mark them with an X if you aren’t going to dop the stones right away, or if you are sawing more than one piece of rough.
Step Seven: Mount the sawn surfaces to dop sticks, of the Moonstone and proceed to grind and polish the cabochons, working through the series of grits from coarse to polish.
By using this method, you will be able to get two matching Moonstone cabochons each having a bright flash of blue across the top surface of the stone.