Cabochons Proportions
PROPORTIONS OF CABOCHONS
The proportions of cabochon gems vary widely, some being quite thin in relation to their width, others being quite thick. Gemstones displaying star, catseye, and moonstone effects must be cut reasonably thick to “sharpen” the lines of light in the case of the first two and to induce strong sheen in the case of the last. Good star and eye effects require that the thickness of the cabochon be from i/3 to i/2 the width; good moonstoone effects are seen with the same proportions but do not lessen greatly until the thickness is reduced to about l/8 the width, depending on how strong the effect is in the rough used. Catseye gems, particularly chrysoberyl catseyes, often are cut with high tops and sharp curvatures at the crests, somewhat elongated along the streak of light, if the stone is elliptical.
The sharp curvature narrows the line of light to a very thin, intensely bright line which enhances the gem’s value. Very shallow curva¬tures must be used on gemstones whose optical displays or patterns of inclusions (e.g., tree agate) appear in planes; sometimes the curvature is so shallow that the top of the cabochon is nearly flat. Gemstones which must be cut this way are labradorite, sunstone, and tree agate. Precious opal is often shaped in this manner, especially black opal, whose rough mostly occurs in thin seam sections to begin with. On the other hand, the trans-lucent Mexican-type opal appears best if the cabochons are shaped as pro¬portionately thick as star gems.
The strength of material is also a major consideration in determining the depth of cabochon gems. For example, weak materials such as mala¬chite and rhodochrosite must be cut fairly thick to prevent breakage during wear, but very tough materials such as jadeite, nephrite, and rhodonite can be cut quite thin. Another important consideration is the total weight of the cabochon, especially if it is large in area. If cut too thick, it may weigh so much that it hangs heavily and awkwardly in a pin or pendant, much to the discomfort of the wearer.