Choosing and Orienting Rough Gemstones for Faceting

Selecting and orienting rough gems can be challenging for new faceters. These tips can help you choose the right tools and resolve some common problems.

Let's go over some tips for choosing and orienting rough gemstones for faceting. We'll cover the different areas and conditions under which you could be checking your rough.

Tools for Choosing Rough Gems

When it comes to selecting the right rough gemstones for faceting, having the proper tools can make all the difference. Here are some essential tools to have in your arsenal:

10x Loupe

A high-quality 10x loupe is a must-have for any faceter. This tool allows you to closely inspect the rough material, looking for inclusions, flaws, and other features that could impact the final cut.

Back Lighting

Shining a light from behind the rough gem can reveal internal structures, color zoning, and other characteristics that may not be visible under normal lighting conditions.

Immersion Fluids

Immersing the rough gem in a liquid with a similar refractive index can help make inclusions and other internal features more visible. This is especially useful for transparent materials.

Calipers

Precise measurements of the rough gem's dimensions are essential for planning the cut and maximizing yield. A good set of calipers is a valuable tool.

Pocket Gem Guide

Having a reference guide on hand that provides information on the properties and characteristics of different gemstone varieties can be incredibly helpful when evaluating rough material.

Orienting Rough Gems for Faceting

Now, let's turn our attention to orienting rough gems. Factors such as cleavage planes, twinning planes, and pleochroism will make you play the orientation game. If you let it, this game can become very confusing and frustrating. Always remember the K.I.S.S. Principle (Keep It Simple, Stupid).

Cleavage Planes

If a mineral has a pronounced cleavage plane or planes, it's important that none lie parallel to the gem table or any facets. If the rough is a nodule, or water-worn, you might see the cleavage plane from the flat face or faces. Usually, these faces will look brilliant, as if they were already polished. Also, look for small groups of gas bubbles or veil inclusions. These usually follow the cleavage planes. Hold the piece under a strong light and rotate it until you see a flash of light. Then, flip it 180°. If you see a flash again, you've just found the plane. Mark it so you can find it again before dopping the stone.

Multiple Cleavage Directions

Now, some materials have multiple pronounced cleavage directions. Many have two, some three, and others four. Then, you have things like sphalerite with six ungodly cleavage planes! No matter how many cleavage planes the material has, just make sure that none lay where you're putting your table facet.

Cleavage Planes in Relation to Other Facets

Over the years, I can't remember having any real problems with cleavage planes when placing mains or break facets on, or close to, the plane. (Except in the case of spodumene). Only when the table runs parallel to the plane do you encounter real trouble, since the table is the largest facet. Often, it gives you a hard time just because of its size. Also, it's the most noticeable facet. Therefore, don't compound it with a cleavage plane. If you're somewhat in doubt, find the cleavage plane on a piece of topaz. Then, try to polish its cleavage plane as if it were a table facet. You'll experience the effect of a cleavage plane in all its glory. Remember, it's better to have the plane on any part of the stone than on the table.

Twinning Planes

A few gem materials (chrysoberyl, for example) can form as twins. The V-notch in the twin is called the re-entrant angle. The bottom of this notch is called the (profanity deleted) twinning plane. Now, this plane is a weak area that can, and likely will, cause the stone to lose brilliance by scattering light. In the worst cases, the stone will split along this plane. Splitting can happen at any stage during the cutting or polishing process without warning. To handle this problem, place the twinning plane parallel to the table and just below the girdle. Although this may cause some loss of brilliance, at least you'll have a stone that will stay in one piece. This is the best solution.

Color Orientation

Color zoning is a band or spot of color deeper in hue than the surrounding material. Sapphire, amethyst, and citrine frequently show this. Simply orient the desired color to the culet and, presto! The desired color will flood the rest of the stone. Bi-colored gems will have two or more bands of entirely different colors in the same plane. You'll see this commonly in watermelon tourmaline and ametrine. Orient these with the color separations directly under the table. Use an emerald cut or other step cut for this purpose. This is the simplest solution. If you use a brilliant cut, whether round or oval, with the colors oriented vertically, they'll get blended. You may find the results unsatisfactory. For example, you could turn a yellow and purple ametrine into brown. Nevertheless, some faceters have cut striking gems from bi-colored rough using brilliant cuts. The trick is to put the color separation parallel to, and close to, the girdle. However, many subtle variations in the gems can affect how well this works. Each gem becomes an experiment in color separation.

Pleochroism

Isotropic minerals show only one color of the spectrum. This applies to colorless gems, too. With these stones, you don't need to orient for color. On the other hand, pleochroism refers to gems showing more than one color. Dichroic gems show two different colors or shades through two different directions of the crystal. Tourmaline is a good example of this. Trichroic gems transmit three different colors or shades through three different directions of the crystal. Untreated tanzanite and iolite are good examples of this. If your rough shows more than one color, you have a variety of priorities to weigh. Normally, you'd want to orient your rough so the best color runs between the table and culet. However, this will often cost you a considerable amount of material. The resulting gem may be much smaller when orienting for color rather than maximum weight retention. If you're choosing between cutting a blue or green sapphire, you'd almost always opt for blue. The value of the finished gem will usually make the weight sacrifice worthwhile. On the other hand, you might have a tanzanite with blue and purple directions. Although blue is the most desirable color in tanzanite, it doesn't have much effect on the gem's value. If you sacrifice too much yield for the sake of color, your cutting may cost you your profits. Here, you want to orient for maximum yield.

Very Distinct Dichroic Colors

Some dichroic rough stones display two very different colors, like purple and yellow. This presents a distinct set of factors to evaluate. Let's say you oriented the table of a purple and yellow tourmaline to the purple axis. If you use an emerald cut, you'll see only purple most of the time. A brilliant cut, with opposing facets close to half way between the axes, will reflect color from the yellow direction as well. The resulting gem will be purple with yellow flashes. This often creates a special effect in the gem that many find highly desirable. Andalusite is noted for this. However, look at how well the two colors go together. If the yellow and purple clash, a brilliant cut makes a poor choice. Instead, use an emerald cut.

Selecting and orienting rough gemstones for faceting can be a complex and challenging process, but with the right tools and knowledge, you can overcome these hurdles and create stunning, well-cut gems. Remember to always keep the K.I.S.S. Principle in mind, and don't be afraid to experiment to find the best solution for each unique piece of rough material.