によってレビュー Andreas Zabczyk Feb 13, 2008 更新しました Feb 06, 2022
Gemstone Shape and Cut: Is There a Difference?
In the gemstone world we are taught that shape and cut are different concepts., but the difference isn't always obvious. Is a trillion a cut or a shape?
Trillion Shape - Plain Cut
The term cushion cut clearly suggests it refers to a cut, but the word 'cushion' is actually shape term, which means shaped like a cushion or pillow. The same goes for 'pear'; it seems obvious that it's a shape, so why do we say 'pear-cut' when we refer to a gemstone in the shape of a pear? It all sounds rather confusing -- maybe trying to distinguish between gemstone shape and gemstone cut isn't such a good idea after all.
Round Shape - Portuguese Cut
Even if our everyday speech is a little loose, there actually is a point to the distinction between shape and cut. Understanding the difference between the shape of a gemstone and its type of cut helps us understand what gem cutting is all about. So let's see if we can make sense out of the all confusion between these two very different terms.
Cushion Shape - Facet Cut
Consider round gemstones, for example. Looking at our inventory of round shaped gems, we can find a great number of different types of round stones; some are cut as cabochons, with no facets at all. Some are cut with a traditional faceting technique, or with a so-called 'brilliant' diamond-cut. Others are cut with very different fancy faceting techniques, such as the concave cut, rose-cut or a buff-top cut. Yet others have what is called a Portuguese cut or even a checkerboard-cut.
So we have one shape - round - and multiple types of cutting styles. Though we sometimes use the phrase 'round-cut', it's quite obvious from our example that this phrase doesn't make a lot of sense when it comes to an accurate description. We need to say round Portuguese cut or round cabochon, and so on for example.
Oval and Pear Shape - Cabochon Cuts
The meaning of the concept of shape should now be fairly obvious. A shape is the face-up outline form of the stone, whether it be round, oval, square, rectangular, pear, marquise or trillion. Given a stone of a certain shape, a gem cutter or lapidary can decide to cut that gem with different styles of facets, or no facets at all.
At the simplest level, we can distinguish the plain cut from the faceted cut. A plain cut has no facets, and may be executed as a level tablet (as in agate) or with a domed top like a cabochon. A faceted cut has a number of separate small planes. There are also mixed cuts which combine the two; for example, a gem with a faceted pavilion and a plain crown.
Round Shape - Brilliant Cut
There are many different facet styles and they can be distinguished according to the shape of the facets. There are three basic types; brilliant cuts, with mainly rhomboid and triangular facets in a radial pattern; step cuts with trapezoid or rectangular facets in concentric rows; and mixed cuts, which combine both brilliant and step-cut type facets. A detailed description of the various facet cuts can get quite complicated and very technical, even for those in the gem and jewelry trade. For example, there are at least six well-known variations on the 56-facet brilliant cut, with different proportions for the facets. Then there are also variations of the brilliant-cut, some with additional facets, while others have fewer facets as well.
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