The Tucson Gem Show is right around the corner and you're probably making a shopping list. Buying stones can be daunting, especially to those new to the business. When considering a stone the first thing we often see is color and pattern. But there is so much more to know when contemplating spending your hard earned money on a stone. In this article we will discuss how determine a high quality cabochon and avoid those painful "mistake purchases".
What is a cabochon? ( kaa·bow·chaan) A cabochon is a gemstone that features convex rounded top which gives it a dome-like shape at the top, whereas the bottom of the stone is generally flat. They may also have a flat top. Side walls may be tapered or straight.
First, educate yourself about the material you're looking to purchase. Understanding the properties like hardness, (Mohs Scale) will help you choose the right stone for your work.
Shape: Considering the shape of a stone is very important. Buying a stone because it is pretty often results in a drawer full of stones you will not use. Think carefully about whether the stone will work in the your designs and how you will set it. Is it even?
Are the lines pleasing?
Types of shapes:
Square and Rectangle
Representational - Heart, Star, Moon etc.
Photo credit Barlow Gems
Note on Stones with sharp corners: When buying these shapes it is very important to pay attention to the corners. Very sharp corners are attractive but if the stone has straight sides they can be challenging to set. It is very important to look at these stones carefully for chips or cracks, more so than rounds as they are more likely to break when setting. If you are new to setting these shapes look for cabochons with softer rounded corners.
Free Forms: This is a very popular cut among designer cutters. They are generally abstract in shape or are soft geometric shapes. All the considerations given to other shapes apply to free form cabochons. Look for pleasing well balanced forms.
Now let's examine the structure of the stone.
Contour: A cabochon should have an even curvature to its surface. Look carefully at the cab from both ends and both sides. The shape (the curvature) should be a mirror image from side to side. No area should be thicker than its opposite. There should be no bulging. Look for flat spots by examining the stone from all angles. Some material lends itself better to a flat top. If the stone is cut with a intentional flat top look for evenness there as well.
Sides- Look at the sides of the stone. A well cut cabochon will have slightly sloping angles with the bottom slightly larger than the top. Typically about a 60° angle on domed cabochons is standard. Some cabochons are cut with 45° straight sides, this is perfectly acceptable and can be fun to set, either in bezel or prong setting. If you are setting your stones a prong setting look for fully polished sides. You may find cabochons that are partially cut leaving a portion of the cab with the natural "ring" edge. Regardless of the cut the sides of a cabochon should be polished.
Thickness: This often ov
er looked and crucial when trying to set a stone. Look at the sides, are the cabochon walls straight or curved? If straight how tall are they? If you are going to bezel set a stone it needs to be the right height for your designs.
Unless you are a lapidary artist as well as a jeweler you will want to buy stones that are the correct height to set. This will vary depending on your bezel height. Super thick stones can be difficult to bezel set as they require a very tall bezel which can over power your design while very thin ones may require a step to get them to the correct height to bezel set. Finally ask yourself can I set this stone? But do consider buying a challenging stone and seeking professional instruction to expand your jewelry making skills.
Edpidote cabochon with flat surface.
Note the beveled edges.
Back: - Look at the back. Is it level and flat? Is the back edge beveled? This is very important as it prevents chipping when setting the stone. Is the back polished? Again if setting with prong or open back settings a polished back is preferred.
Polish: When examining a cabochon, look closely at the polish. Look for any scratches or pitting that will reduce the amount of light reflected from the surfaces. Move the stone so the light travels across the top. If the surface is properly cut, you’ll see the band of reflected light glide evenly over its surface. The band of light will begin to snake if there are any irregularities.
Types of Polish: The final surface treatment on a stone. This can vary depending on the material and even then polish can vary greatly in a single stone category.
High Polish: Material with a high hardness (Mohs scale) will take a very high polish. These include agate, chalcedony, jade and many jaspers and petrified woods. However turquoise can be highly polished as can ivory. Most jaspers will polish very well. Glossy Polish: Semi gloss finish. Some materials are soft making it difficult to get a super high polish, these include dolomite, serpentine, maligano jasper and a plethora of other materials. Although exceptions do apply.
Matte finish. Duller finish that often enhances the color saturation of the stone. Many stones are quite interesting with a matte finish. Materials vary.
Natural Surface: This is when the original surface or the stone is left untouched. Some material like Lapis, Aquamarine, Tiger Eye are quite beautiful with a naturally cleaved surface.
NOTE on polish: Not every material will polish to a mirror finish. Understanding the material you are looking to purchase will help you make a good purchase. If you are not sure about the stone you're buying ask a professional.
High Dome Cabochons: High domes are cabochons cut very tall and the dome is often quite curved. This cut is done to capture the light in the stone. Some materials generally cut into high domes include: Chalcedony, Chrysoprase, Gem Silica Chrysocolla, Carnelian, Citrine, Aquamarine, Tanzanite, Tourmaline, Ruby, Sapphire and included Quartz. Some opaque material like turquoise can also be found in high dome cuts.
TIP: Run your finger or fingernail over the surface of the stone to identify cracks. Often cracks will not be eye apparent.
Taking the time to really look at the cab you are buying is very important and will save you money in the long run. As we know this addiction is very expensive.
Enjoy shopping at the largest Gem and Mineral show in the World!