Color zoning, or color banding, is a term used to describe a natural color distribution that occurs in almost all gemstones, but is most common in tourmaline, quartz and sapphire. (This is not to be confused with pleochroism, which is the change of color due to different viewing directions – a topic for a future blog post!) The most common cause of zoning is the availability of trace elements during the crystal’s growth, sometimes producing variations in a single color’s intensity, and other times producing totally different colors. It is considered beneficial when two or more colors are clearly visible and show striking patterns. This is the attraction of ametrine – a mix of purple and yellow quartz, and of multicolored pink and green “watermelon” tourmaline. Sapphires that show zoning are known as “parti-color” sapphires. Some blue sapphires (especially some from Montana or Australia) have yellow growth areas, producing a parti-colored stone with shades of blue, green and yellow after cutting. Pure blue/yellow bi-color combinations are rare in sapphire, as are other such distinct color combinations. These unique sapphires are not available commercially and are perfect for those looking for a gemstone that expresses their individuality.
Color change is one of the classic gemstone “phenomena”. Some of these gems display a subtle color shift when exposed to different types of lighting such as daylight vs incandescent lighting. For example, a garnet that appears pinkish-red in daylight may look purplish-red in indoor lighting, but since red is the same basic hue it’s not considered a true color change. A true color change entails a change in the hue (think of the distinct colors in a rainbow – purple, red, orange, yellow, green, blue and violet). Examples include the green to purple change in an alexandrite chrysoberyl, or the blue to purple change that’s somewhat common in Ceylon sapphires. It’s thought that the elements vanadium or chromium play a role in most color change gems, causing a slight alteration to the gem's crystalline structure and thereby affecting the color absorption of the light transiting the gemstone. Besides chrysoberyl and sapphire, color change phenomena can sometimes be found in diaspore, spinel, garnet and tourmaline. We love color change - it's like two gems in one!
Peridot is a type of silicate mineral known as olivine, and is most often formed underground and brought with the surface with volcanic activity. In Hawaii, peridot has historically represented the tears of Pele (the volcano goddess of fire) and very small peridot grains can be found on some Hawaiian beaches. Peridot can even be found in some pallasite meteorites! While the mineral olivine is quite common, gem quality peridot is rare. Today the primary source is the San Carlos Reservation in Arizona, but other sources include Burma (Myanmar), China, Tanzania, Sri Lanka, Afghanistan, and more recently, Pakistan, where large flawless crystals with excellent color have been found. The finest color is a rich vibrant green with a slight yellowish tint known as “apple green”. Peridot is an affordable and popular gemstone, and because it has a Mohs hardness of 6.5-7 it can be used in all forms of jewelry.
Did you ever wonder why two similar gemstones often show a difference in brilliance, scintillation and dispersion, even with the same material and same cut? Commonly known characteristics like cut, clarity or refractive index are all important, but it’s a little understood characteristic called luster that plays a critical role. What is luster? Luster represents quality of the polish (reflectivity), which depends on both the material and the polishing agent. Luster is often confused with brilliance, but these are two completely different characteristics. Generally speaking, hard materials such as metallic compounds or diamonds have the highest luster, and soft materials such as amber have low luster. Pearls have, of course, a “pearly” luster. A high-quality polish facilitates refraction and dispersion at a molecular level at the gem’s surface, subtlety but critically improving the appearance of the gems.
There are two major groups of polishing agents – oxides and diamond. Oxides such as cerium, alumina and chromium have their use for specific purposes (quartz is commonly polished with cerium oxide), but most other stones are successfully and quickly polished with various grits of diamond. Large diameter diamond grits such as 320 or 600 are used for rough cutting, fine grits such as 3000 or 8000 are used for pre-polish, and extra-fine grits from 50,000 to 100,000 are used for polishing. 14,000 is often used as a final polish in overseas cutting houses because it’s fast, but it leaves fine scratches and a “greasy” look on the facet surfaces that can inhibit refraction and dispersion. A 100,000 polish takes more time and skill, so it’s often used only for gemcutting competitions. Many cutters will say that customers can’t tell the difference in polish, but we disagree! All Finewater-cut stones except quartz are polished with 100,000 grit diamond, so you can always be assured of the finest possible polish and luster.
For this post we will take a look at faceted vs rounded girdles and their effect on the appearance of the gemstone. Most "western" cutters learn gemcutting by following diagrams produced with the gemstone design software called GemCad. It's a fantastic tool, but one of the limitations is that it produces cutting instructions with straight faceted girdles instead of a smoothly rounded girdle on such shapes as rounds, pears, ovals, etc. Most common gemstone shapes cut with a strict adherence to GemCad instructions result a total of 16 such girdle facets, producing a "16-gon" mathematical figure called a hexadecagon, rather than a true round, oval or pear shape.
There is always some artistic license when cutting stones and for some fancy cuts a faceted girdle may be required, as it is for squares, octagons, rectangles, hexagons or other shapes with straight sides. Many new cutters believe that faceted girdles are a sign of a "precision cut" gemstone, but a rounded girdle can also be cut and polished in a precision manner - it just takes a little more time and effort to produce what we think is a more refined look. You would never see a ideal-cut diamond or an high-value colored gemstone with faceted girdles; you shouldn't need to settle for these either.
In addition to the aesthetic aspect, there's also a functional reason to round the girdles. Many jewelers hate faceted girdles because it makes stone setting more difficult. A rounded girdle also facilitates the use of settings with any number of prongs, with no need for awkward fittings such as a 6 prong setting on a 16-sided stone. A rounded girdle also works better with a bezel setting, as the metal can then be pushed into place more smoothly around the girdle.
Here are a few examples of faceted and rounded girdle outlines, with the GemCad-type faceted girdles on the left and rounded girdles on the right.
We think you will agree that the stones on the right appear more elegant. It shows that a little extra effort can make a world of difference!
People often wonder "what makes a good gemstone" so I've decided to write a series of short blog posts about various gemcutting or gemology topics - in no particular order. Here's the first installment, showing an easy way to visually determine a cutter's commitment to detail by examining a gemstone's crown facets.
Most western cutters learn what's called meetpoint cutting - that is, cutting each facet until it precisely "meets" or joins adjoining facets. When done properly it is a work of beauty; when not, not so much. Some cutters will take shortcuts, particularly when it comes to the crown facets that meet the table (the "star" facets on a round brilliant cut). In contrast, most foreign cutters will cut a good looking crown because that's what the customer will notice after the stone is mounted. We've found that it really doesn't take much additional time or effort to finish the meetpoints correctly.
Here are some examples (both good and bad):
So you see that's really quite easy to judge some aspects of cut quality from a quick examination of the gemstone (and often a photo will suffice). Of course, there will always be some exceptions where cutting constraints that might prevent perfect meetpoints - for example, repolishing a badly cut stone that had poor symmetry or to avoid inclusions just under the table. However, for the majority of stones (and especially those cut from rough) good table meetpoints should be the norm rather than the exception.
On each of my Sri Lankan buying trips I go with good intentions to purchase poorly-cut sapphires for recutting, yet it's becoming more and more difficult to find stones with a combination of poor cutting and top color. Sri Lanka has a long history of cutting, and while some stones are still cut on traditional hand-powered machines, most are now cut using modern precision machines. I'm often impressed with the innovative designs and cutting quality seen in the gem markets. As a cutter myself I appreciate that there's a consideration for the extreme rarity of clean facet-grade rough, meaning that they are cut for brilliance as well as weight retention. It is definitely possible to do both!
I've had a couple of recent trips to Myanmar (formerly know as Burma), and found lots of nice spinels in all colors but I focused my buying on the nice reds. The reds are outstanding due to the incredible fluorescence of these Burmese stones. Under UV light they have an unreal glow to them (see photo below); in normal daylight the fluorescence ramps up the saturation and intensity of the color. It's too bad that many of the stones that I saw were so poorly cut, but I did buy a couple of poorly cut (but deep) stones to recut because they were clean and had top color. Some rough spinels were offered including the awesome crystal shown in the photo below, which I would have bought except the owner was expecting an insanely high price. Maybe on my next trip I'll see some reasonably priced material.
I've just returned from annual buying trip to the Sri Lanka gem markets with nice sapphires, spinels and garnets. It's always cool to be one of the few Westerners to visit these markets where I get to see great stones, with many of them new to the market. As expected, there are plenty of sapphires to choose from, but there are always interesting stones such as cat's eyes, alexandrite and rare stones like sinhalite and andalusite. In addition to locally-sourced stones I also see many from East Africa, as Sri Lankan buyers purchase rough in those countries for cutting in Sri Lanka.
There were some opportunities to explore the area around the gem markets, so I saw the traditional blowpipe heating, gemcutting and rough stone markets. As a cutter, the rough stones were fascinating, but since rough stone exports from Sri Lanka are prohibited to protect the local cutting industry I couldn't bring any home.
I was also honored to be invited to speak at the 2016 International Gemology Academy conference this year, with my presentation on Gemstone Photography Techniques. The presentation was very well received, and I made many important contacts as a result. My broker tells me I'm now famous in the local community! I can't verify that, but I was introduced to several industry contacts including some dealers in Colombo that specialize in world-class sapphires. If you've ever wanted a top sapphire (like the 10 ct sapphire shown below) please let me know - I can probably find it for you.
I've just returned from a fantastic trip to Namibia, a place of stark and unparalleled beauty. Namibia is a mineralogical hotbed and is well known for such gemstones as diamonds, demantoid garnets, amethyst, tourmaline, and unique mineral specimens such as peitersite. It was great fun driving the remote back roads along the Skeleton Coast and in the Spitzkoppe and Erongo mountain districts, watching out for wildlife, poking through thousands of mineral specimens at the roadside shops, and searching out gemstone dealers and prospectors in the few small towns. My trip produced some interesting material, and new stones will begin to show up on my website in the coming months. If you haven't done so already, please sign up for my newsletter in order to get notification of these new stones.