How a Cutter Decides Between a Facet and a Dome

A cutter holds a piece of rough and decides between a faceted stone and a cabochon based on what the material can do, because the wrong choice wastes both color and weight.

Transparency comes first

Faceting needs light to pass through the stone. If the rough is transparent and relatively free of internal features that scatter light, faceting returns that light through the crown and produces brightness. If the rough is translucent to opaque, very little light enters, so there is nothing for facets to return, and a dome is the better use of the material.

There is a middle ground. Translucent material with strong color is sometimes faceted with shallow proportions to keep the color from going dark, and the result can look fine in the hand while showing a window in bright light. That trade is a decision, not an error, and it is worth knowing which you are buying.

Optical effects need a dome

Stars, cat's-eyes, play-of-color, and adularescence depend on the curved top of a cabochon to bring their reflections together at a single line or point.

Orientation matters as much as shape. A star sapphire must be cut with its crystal axis perpendicular to the base so that the rutile needles meet at the correct angle and the star sits centered. A cat's-eye must be cut with the fibrous inclusions running parallel to the long axis of the dome. Get the orientation wrong and the effect is weak, off-center, or missing entirely.

This is why a sapphire full of fine silk is more valuable as a cabochon than as a faceted stone. The same needles that make a star lower the clarity of a facet and are better left exposed on a dome.

Weight, and what gets lost

Cutting removes material, and faceting removes a great deal of it, because the cutter has to establish angles, a table, and a girdle. A dome can preserve far more of the original weight, which is one reason a heavily included or oddly shaped piece is often cut as a cabochon.

The arithmetic is not only about weight. Two finished stones can come from the same rough with different trade-offs: a larger stone with weaker color, or a smaller stone with better color and cut. Neither is wrong, and the one that sells for more depends on the species and the market.

Cutters also remove flaws when they can. A fracture near the surface may be ground away entirely, or it may be positioned under the girdle where a setting will hide it. That is normal practice and it is also why the side and back of a stone matter: a photograph taken straight on will not show you where the inclusions ended up.

Orientation and color in pleochroic stones

Many interesting species show different colors from different directions. Tourmaline, tanzanite, iolite, and kunzite all do. A cutter orients the rough so the best color shows through the table, and may cut away the darkest end of a tourmaline crystal, where the color is so saturated that the stone would look black.

Recutting an older stone

An old stone may have a damaged facet or an unfashionable outline, and a recut can fix both. It also reduces weight, sometimes substantially, and it can remove a feature that gave the stone character. A recut also changes the stone's identity for documentation purposes, which matters if it carries a laboratory report or an appraisal.

Look at the outline for symmetry, the girdle for thickness, and the middle of the stone for a window or a dark center, and ask for side and back photographs, because proportions tell you more about a cut than a straight-on view of the table.