Many of us choose rough without thinking of designs and wonder later on about how to cut the material. What shape should we use? Will the selected shape accommodate to the rough? What kind of return will we get from our purchase? While the answers to these questions are rarely straightforward, our presentation this evening relates to correlating designs with rough gemstones.
The range of shapes one can cut stones into is quite wide. We’ve included a group taken from the DataVue collection: Index to Facet Designs: Shape Classification List.There are forty-six shapes and thousands of designs available both within and without DataVue. Some years ago, Michelle Heath presented a description of Free-Form and Fantasy Cuts, allowing many more choices. However, tonight we will look at samples of the rough you brought in and we’ll consider the alternative cuts available for each. Hopefully, after we have shared ideas, we’ll all go home with a better idea of our choices when we look at a piece of rough.
In the January Newsletter, Bill very thoughtfully included an article by Howard R. Bromley, How Much Rough Do I Need? The writer explains how one can choose a design and then estimate the size and shape of the rough needed.
When buying rough material, one needs to consider many criteria, all ready discussed during earlier meetings:
- Kind of material
- Cleavage plane(s)
- Colour and colour zoning
- Doubling of facets
- Orientation of the crystal axes
These aspects must be considered when selecting a design to cut.
GemCad is a most helpful tool!
- Each diagram has several measurements included enabling one to make a decision about what design will give an optimal return on your rough.
- L/W, H/W, U.W, P/W, C/W and Vol. /W^3 = ? 
The volume of the stone divided by the cube of the width of the stone is equal to a figure, for example, 0.279. [The cube of a number is the number multiplied by itself three times.] The volume fraction can be used to estimate the carat yield of a finished cut.
The width of the stone is shown on the GemCad screen and measured either horizontally or vertically when viewed from the top, whichever is smaller. To use the volume fraction to estimate carat weight, measure the width with a caliper in centimeters (mm/10), cube the measurement and multiply the result by the specific gravity of the material in g/cc in turn multiplied by the number from the volume fraction (volume / width cubed) given by GemCad. The result will estimate the weight of the stone in grams. Multiply by 5 to determine the weight in carats.
Width (mm/10) x W x W x Specific Gravity (g/cc) x Volume = Weight in grams x 5 = Carat weight
For example, let’s compute the weight of a 12 mm wide piece of quartz with Volume/W3= 0.279. Noting that quartz has a specific gravity of 2.65 g/cc, and 12mm = 1.2cm
Weight = 1.2 x 1.2 x 1.2 x 2.65 x 0.279 = 1.28 grams x 5 = 6.39 carats
If you prefer, you can use millimeters instead of centimeters; the weight will come out in milligrams. You then divide by 200 to get the weight in carats. Using the figures above,
Weight = 12 x 12 x 12 x 2.65 x 0.279 = 1280 mg / 200 = 6.39 carats
Note that the volume fraction depends on the actual girdle thickness shown in the diagram, so it will vary with the girdle thickness shown on the screen.
- Use the C command in Gem Cad to round off the indices.
The above calculations are simplified in a downloadable program written by Gary Kratochvil: Gem Weight Estimator.
- 70 % of the Width will give you the estimated Height (depth) of the stone. A 5 mm stone will have a depth of 3.5 mm.
- This is particularly important when cutting dark material such as dark Blue Sapphires.
Important Factors to Consider When Selecting Which Design to Use:
Make sure to have available: white paper, micrometer or calipers to measure various dmensions. Some designs by A. Wolkonsky for example, often measure across the corners of the stone (sharp cornered stones) in order to obtain the exact size.
Other helpful instruments which were shared during the evening included:
- A map grid originally made by the Canada Map Company with which one can examine the dimensions of a piece of rough and get an idea of possible cutting shapes.
- A lever-based protractor allowing consideration of a variety of shapes.
When examining gemstone rough, find the C-axis of the material to determine doubling of the facets perpendicular to it. Examples include: Peridot, an extreme case, or Kyanite.
Color is important in dealing with tanzanite.
Consider a choice of design in terms of its use in jewelry. Remember that deep stones are not easy to design for, or set in jewelry.
One possible approach is to measure the rough and draw it on a piece of paper. Determine the critical angle. One can then estimate crown and pavilion angles and determine if the rough is suitable for your design.
Return on rough based on the shape of the selected design:
Round Brilliant – 24%
Oval – 33%
Marquise – 15%
Pear – 13+%
Sq. Step – 40%
Emerald – 33%
The Round Brilliant is one of the more wasteful of material of all the designs.
With the Emerald Cut one needs to dispense with the normal rules of centering your stone or cutting at a set platform height:
When performing an emerald cut on high priced material first, cut one side of the crystal mirror image and on the opposite side one may have to measure the width of the facets in order to obtain the greatest return on your rough. Facet an even girdle to facilitate cutting an Emerald Design.
The bow-tie effect is frequent especially in Marquise Cuts; if the L:W ratio is greater than 20% or 1:1.2 that effect will be more visible. The same result will be more pronounced in barion designs.
The remainder of the evening was devoted to examination of the rough material brought in by members. We realized a number of things as members of the group helped each other with design choices or were assisted by the authors.
- It is helpful to have a good three-dimensional perspective when regarding stone and rough.
- We rarely found suitable shapes without consideration of the items noted above when buying rough, i.e., inclusions, color zoning, fractures, cleavage, crystal orientation, etc.
Thus, a tablet (flattish) piece of beautifully colored ruby required dispensing with the traditional design in favor of a table surrounded by a hexagonal cut single set of facets highlighting it.
A fine piece of pale salmon-colored sunstone was considered in favor of color, not schiller and some optional designs were recognized.
A large piece of citrine offered a material-sparing pear cut rather than a more wasteful (based on the shape of the stone) standard round brilliant design.
Some fine tourmaline invited emerald cuts with consideration of the light and dark c-axial view.
The authors wish to express their heartfelt thanks to Al Manestar for his help and guidance in preparing this presentation.
List of Works Consulted
- Strickland, Robert W. (1992). GemCad User’s Manual. The Diagram Menu. Version 4.51.
- Giesbrecht, Robert. Email to Faceters’ Digest. April 24, 2002.
- Goodger, Don. Personal Communication, 1989.