The Secret to Professional Symmetry

The hallmark of a professional lapidary artist is not just a high polish, but perfect, pleasing symmetry. A cabochon with a lopsided curve or uneven shoulders looks amateurish, regardless of how beautiful the stone's pattern is.

Furthermore, if you intend to sell your cabochons to jewelers or mount them in pre-made silver settings yourself, you must cut them to exact, standardized millimeter dimensions—a practice known as cutting calibrated stones.

To achieve perfect symmetry and calibrated sizes, you cannot "eyeball" the cut. You must rely on lapidary templates, marking scribes, and strict preforming discipline on the grinding wheels.

1. The Tools of the Trade

Before you touch the grinding wheels, you need the right layout tools.

Lapidary Templates

Templates are rigid plastic stencils featuring a variety of cut-out shapes: ovals, circles, teardrops, squares, and crosses. The most important feature of a professional template is that the shapes are accurately labeled with their millimeter dimensions (e.g., a 30x22 oval means 30mm long by 22mm wide). Always use translucent templates so you can see the stone's pattern through the plastic to "frame" the most interesting part of the rock.

The Aluminum Scribe

Never use a Sharpie, pencil, or pen to draw your template outline onto the rock. The water and oil used on the lapidary saw will instantly wash the ink away. Even worse, the ink can bleed into microscopic fractures in the stone, permanently ruining a valuable piece of rough.

Instead, use an aluminum scribe (also called a brass pencil or marking stick). When you draw with soft aluminum on a harder stone like agate, the metal rubs off, leaving a bright silver line that is completely waterproof, oil-proof, and will not soak into the stone.

2. Drawing the Outline (The Layout)

  1. Clean the Slab: Ensure the slab of rock is completely clean and dry.
  2. Find the Pattern: Slide your translucent template across the slab. Look for interesting patterns, avoiding any visible fractures or pits.
  3. Trace the Shape: Once you've framed the perfect area, hold the template firmly so it does not slip. Trace the inside of the shape with your aluminum scribe, keeping the point tight against the edge of the plastic.
  4. Trim the Slab: Take the slab to your trim saw. Cut as close to the silver line as you safely can without actually cutting into the line itself. The closer you cut with the saw, the less time you will waste grinding on the 80-grit wheel.

3. The Preform Stage: Grinding the Outline

The blank piece of rock you cut from the saw is called a preform. The next step is grinding the preform to match the template shape exactly. This is done exclusively on your hardest, coarsest wheel (usually the 80-grit steel diamond wheel).

The Perpendicular Grind

  • Keep the stone perfectly flat and horizontal.
  • Push the edge of the stone straight into the 80-grit wheel at a 90-degree angle.
  • Slowly grind away the excess rock until you reach the very edge of your silver aluminum line.

Do not attempt to dome the top of the stone yet! The goal of the preform stage is simply to create a flat puck of rock with perfectly straight, 90-degree walls that match the template shape perfectly.

Checking for Symmetry

Stop the machine and wipe the stone dry. Look directly down at the face of the stone.

  • Are the curves of the oval perfectly smooth?
  • Does the stone bulge slightly on one side? If you see any flat spots on the curve or uneven shoulders, correct them on the 80-grit wheel now. Once you begin doming the stone, it is incredibly difficult to fix an asymmetrical outline without ruining the dome's geometry.

Grinding the Bevel

Once the outline is perfect, tilt the stone to a 45-degree angle and grind a small bevel around the entire bottom edge (the girdle) of the stone. This crucial step prevents the sharp bottom edge from chipping during the later polishing stages.

4. Moving to the Dome

Only after the preform has a perfectly symmetrical outline, 90-degree walls, and a beveled bottom edge should you begin grinding the dome.

By separating the "shaping" phase from the "doming" phase, you ensure that your final cabochon will always fit perfectly into a standard jewelry bezel setting, drastically increasing the professional quality and monetary value of your lapidary work.