Gem Buying: 5 Ways to Understand Gemstone Cuts
If you’re new to the world of gem buying, you’re probably overwhelmed: by the beauty, the variety of types, colors, sizes, and shapes. The more stones you see, the more you realize that some of the shapes are . . . well, what are they? It’s hard to confuse a round and an emerald cut, but some of the other gemstone cuts? If you want a specific shape, what shape is it that you want? At a booth in Tucson, all you have to do is say, “I want that one.” But if you want to duplicate that cut shape or style later, how do you ask for it?
ABOVE: Three indicolite gemstones (blue tourmaline). The center stone is a 3.52-carat brilliant cut, cushion shape. The two smaller stones (totaling 1.34 carats) are square step cuts. Photo Mia Dixon, courtesy Pala International.
Spoiler alert: There are no clear answers. I’ve talked to a lot of knowledgeable gem dealers, and even they have a difference of opinion. For example, what is a cushion cut?
Gemstone Cutters and Dealers Describe Cushion Cuts
New Mexico cutter Nancy Attaway: “I think a cushion cut rectangular gem could have the long sides bowed, while the short sides may be bowed or slightly bowed, or [the short sides] may be straight and not bowed.”
California cutter Glenn Klein: “It’s almost like an oval that’s been squashed a bit. No definite corners. At first glance you would think it’s round. [It has] sides that are curved slightly or dramatically, not two sides that are curved and two that are straight across.”
New Hampshire-based gem dealer James Alger: A cushion cut is a square or rectangle with curved or arced sides. “But it is a little tricky. Sometimes there are certain stubby ovals that we send to the lab as ovals, and they come back described as cushions and vice versa.” Some people refer to a cushion cut when they really want an antique cushion cut—a cushion with a pillow shape, “with round corners and arced sides.”
New Hampshire cutter John Bradshaw of Coast-to-Coast Rare Stones: “Take the shape outlines of a rectangle or emerald cut and an oval—a cushion is anywhere between those two.” You can have a cushion with sharp corners, or you can soften the corners by changing the shape of the facets in the corners, much like you cut the corners of a rectangle to soften the cut into an emerald cut. “There are no sharp, measurable angles that you can determine on a soft-corner cushion.” When you describe the cushion shape you want, ask for one with sharp corners or soft corners. “That’s about as technical as it gets.”
To confuse the discussion more, Klein mentioned a pentagon cushion, and Attaway mentioned a triangle cushion. What makes either of these gemstone cuts into cushions? They have curving sides, not straight.
Finding the Gemstone Cuts You Want
If even those who handle and cut gemstones every day have slightly differing views on what makes a particular cut what it is, how can a novice, shopping for a stone cut in a specific way, be sure you’ll get what you want?
1. Vive la difference in gemstone cuts.
First understand that different people have different ideas about what a cut looks like. Learn what those opinions are. Look at cut diagrams in books. Go online and look for diagrams there. Go to the shows, look at the gemstone cuts, ask what they’re called. And don’t take one person’s opinion. Ask everyone: “Is this a cushion? Is this an octagon?” Is this a radiant?” You’ll begin to get a feeling for the consensus of nomenclature. You’ll also begin to spot the dealers who use the same terminology that you do. That will make ordering from them much easier.
2. If you don’t know them already, learn the parts of a cut gemstone.
Gems have a table (the large facet in the center of the gem, looking down on it), girdle (the midline of the stone), crown (the top of the stone, above the girdle), pavilion (the bottom of the stone, below the girdle), culet (the usually small facet on the lowest part, the point or the keel line of the pavilion), shoulders (on pear shapes and hearts, the curving part of the girdle outline near the top of the heart or the near the broadest part of the pear shape).
The term corners should be obvious, but if you’re new to all this, the term “cut corners” may not be. Draw a rectangle, take scissors and clip off the pointed corners. These are cut corners. Even cut corners can be softened by adding extra facets to them.
Know that the “depth percentage” is the measurement of the stone from the table to the culet, expressed as a percentage of the diameter of the stone (smallest or narrowest diameter in the case of fancy gemstone cuts with unequal side lengths). So a rectangular cut stone measuring 10 x 8 mm, with a depth from table to culet of 4 mm, has a total depth percentage of 50 percent. (The diameter or width of the stone is 8 mm, and 4 is half or 8, equaling 50 percent.) Crown height and pavilion depth are also expressed as percentages of a stone’s diameter.
3. Learn the difference between cut shape and cutting style.
Cut shapes include round, oval, marquise, cushion, and emerald cut. Cutting styles include brilliant cut, step cut, and mixed cut. These are illustrated in most books on gemstones.
A step cut is one in which the facets are placed parallel to the usually square or rectangular girdle of the stone. The facet angles gradually change as the facets “step” toward culet or table. An emerald cut is a step cut.
A brilliant cut is one in which the facets tend to run from the culet or table toward the girdle. The shapes of the facets tend to be triangle-, kite-, or diamond-shaped.
A mixed cut, as you might guess, combines these two cutting styles—step cut on the pavilion and brilliant cut on the crown or vice versa. A cushion shape (which refers to the girdle outline) can be brilliant, step, or mixed cut. Become familiar with cutting styles by looking at stones at shows.
4. Learn about the physical and optical properties of gemstones.
A gem’s physical and optical properties can affect which shape and cutting style a cutter chooses for a stone.
A stone’s pavilion depth is determined by the refractive index of the gemstone material. (Refractive index, or RI, is the measure of the extent to which light bends when passing through a transparent material.) “The lower the RI, the deeper the stone has to be to maintain optical quality unless you want to ‘window’ the stone,” says Bradshaw. (When the pavilion angles are too shallow, light entering the stone does not reflect correctly from the pavilion facets. You can look through the stone, hence the term “window.”)
5. Talk to cutters about their art.
Ask how the choice of cutting shape and style can save weight from the rough, how it’s determined by the shape of the rough or the concentration of a color in a crystal, how the choice allows them remove ugly inclusions from the rough, add brilliance to a stone, lighten a dark stone, or darken a light one. All these considerations affect the cut shape and cutting style of the stone—and the final weight, which may affect the price.
For example: green tourmaline. Green tourmaline grows in long, often slender crystals. In addition, the color of green tourmalines is often very dark looking down the length of the crystal; the color is lighter and more attractive when viewed from the side. This is why so many green tourmalines are cut into emerald cuts. They follow the shape of the crystal, so more weight is retained in the finished stone, and it results in lighter colored, usually more attractive stones.
However, when cutting pink tourmaline or rubellite, cutters want to take advantage of that darker concentration of color. Rubellites are more often cut into rounds or “triangle cushions” which closely follow the cross-section shape of the rubellite crystal.
Understanding Gemstone Cuts
There are so many factors at play in gemstone cuts, it’s probably safe to say that gemstone cutting shapes will never be standardized. But you can improve the chances of getting that perfect cushion cut, or whatever stone you want for your designs, if you ask questions. Does it have soft or pointed corners? Is it more rectangular, more square, or more oval? Is it a step cut, brilliant cut or mixed cut? If mixed, is it brilliant on top and stepped on the bottom or vice versa? How large is the table? How deep is the pavilion?
You’ll improve your chances even more if you do what James Alger does when having stones cut: send a diagram.
Sharon Elaine Thompson is a GG and FGA who has been writing about gemstones and jewelry for Lapidary Journal Jewelry Artist since 1987. She also writes a line of birthstone romance novels under the name Liz Hartley.