How to Cut Star Sapphires

Sapphires come in many shades of blue and purple.

Star sapphires are one of gemology's most prized treasures. Even more valued than a standard sapphire, star sapphires display asterism, which is an optical effect caused by rutile inclusions within the mineral structure. The rutile crystallizes with hexagonal symmetry around the c-axis of the sapphire; light reflecting from these inclusions form six-ray stars. To enhance this effect, star sapphires are always cut en cabochon, meaning that the finished gem is not faceted, but rounded and dome-shaped. The gem must also be cut so that the c-axis is perpendicular to the base of the sapphire.

Things You'll Need

• Star sapphire stone
• Dopping stick
• Dopping wax
• Grinder or saw
• Suede cloth
• Aluminum oxide

Instructions

The Cutting Process

1. Orient the sapphire so that the c-axis is perpendicular to the plane where you plan to cut the base of the gem. You can do this by visual examination. When the effect of the star is clearest, you are actually "looking down" the c-axis of the crystal.

2. Cut the base of your sapphire with a rock saw or grinder, keeping the base perpendicular to the c-axis.

3. Secure the sapphire to a dopping stick using dopping wax. The stick should be attached to the gem at its base, which keeps the sides and top clear for you to shape.

4. Trim the base of the gem into the desired shape by cutting or grinding away excess material. This gives your sapphire a "footprint" that is the same shape as the setting it will be placed in. Ovals and circles display asterism better than other shapes.

5. Cut the sides of the dome shape using a technique called "peeling the apple". Starting near the base of the gem, roughly three-quarters to two-thirds of the way down the side, grind a bevel all around the gem at a forty-five degree angle toward the center. Follow this with another bevel slightly higher up, and at a steeper angle. Continue to make grinding passes higher and higher up the sides of the gem at steeper and steeper angles, until you reach the center.

6. Smooth the shape of the sapphire with the grinder. Use 600-grit silicon carbide belts or a 200, 600, 1200 series with diamond. Stop occasionally to evaluate the shape of the stone as you grind, and continue until no scratches remain and the shape is a smooth, even dome with no bulges or uneven areas.

7. Polish your star sapphire with aluminum oxide, using the soft side of a piece of suede.

Tips & Warnings

• According to the International Gem Society, "peeling the apple" is the most challenging step for beginners, particularly as you approach the center of your gem. Be patient and make more than just a few passes; this perseverance will ensure no flat spots or bulges in your cabochon.

• Any imperfections that are not removed during the smoothing process will remain after polishing, so be meticulous when smoothing to make the shape symmetrical and even, and the surface scratch-free.

References

• Gemstone Education: Star Sapphire
• International Gem Society: Fundamentals Of Lapidary Part 4, Cabochon Cutting