Handling Colored Stones at the Bench Part 1
9 Minute Read
Ever since childhood, I've been fascinated by rocks and gems. I started cutting cabs more than 40 years ago, and have been faceting colored stones for about 30 years. I do precision cutting of colored gems these days, and that often includes repairing or re-cutting damaged stones. If I dig around I can find some certificates and degrees that allow me to put my initials after my name, but, like most of you, the real learning has to come at the bench. And, I'll bet like you again, the lessons remembered best were the ones that cost me money and embarrassment.
I'm hoping this series of articles will save some of you time, money, and embarrassment by discussing some of the common bench procedures that can easily lead to damage of colored stones - and how to avoid it. Some of it will be familiar, some of it new, and some of it controversial. At the end of the series I'll be offering a concise guide to help with bench related questions about colored stones. In the meantime, you can call or e-mail me anytime with your colored stone questions and I'll try to help.
Basically, I'll discuss about 40 varieties of faceted stones and another 40 or so different lapidary materials. This is not meant to be a gemological discussion; rather it's intended for the non-gemologist, for the bench person and even the sales person who works with the public.
Let's start off with some basic guidelines by talking about the use of steamers in the shop. Steamers were designed for rapid, high-pressure, high-temperature cleaning of metal objects, and they can do a tremendous job. They can also instantly destroy many colored stones, and that includes sapphire, ruby, certainly emerald, and yes, diamond. You will not find steamers in diamonds cutting houses for good reason! Crystalline objects often do not respond well to sudden changes in temperature, and large diamonds have been known to shatter when placed under a steamer. Now I know that most of you have used a steamer for years with no problems, or so you thought, but please consider the risk involved versus spending just a minute or two using a less traumatic cleaning method. Much of the damage caused by a steamer will go unnoticed in the hurry of the shop, but when the customer sees her nine carat orange garnet full of cracks after your cleaning job, you have a very expensive problem! My advice throughout these articles with regards to steamers is the same. Don't use them. Judicious use of a good ultrasonic cleaner with reasonable fresh cleaning solution followed by a dip in alcohol will clean as well as or better than any steamer, with much less risk. That's my story and I'm sticking to it!
Most ultrasonic cleaners are heated; mine actually steams, so it's quite hot. Before placing any stone-containing jewelry into a hot ultrasonic, proper procedure includes warming the piece first. I have a plastic cup by my steamer with about an inch of tap water in it. I add another inch or two of hot fluid from the ultrasonic, drop the piece in the cup for 20 seconds, and then place it in the ultrasonic, successfully avoiding thermal shock and angry clients.
The last guideline concerns removing stones from their setting before commencing work involving heat. The decision requires judgment on your part, but I suggest you err on the side of caution. Your client may have signed a "hold-harmless" agreement as part of the take-in procedure, but if you use high heat near any colored stone and damage it, you may well wind up dealing with an attorney who claims you were negligent. You can't go there, so be smart.
General Handling Rules for the Bench
Always err on the side of caution. Remember that the stone may not be properly identified on the job envelope, so if you're not sure what it is, treat it as very expensive glass.
If the stone is to be removed, assume it is brittle and use proper prong pullers or saw the prongs off. NEVER pry between abezel or prong and the stone.
There is a big difference between hardness and toughness. Although diamond is the hardest substance, even a slight strike with a small hammer can shatter a diamond, while jadeite, which is technically much softer, will hardly be bothered. But please don't test this on a client's piece!
Remember that while many stones can survive SOME heat, sudden changes in temperature from torch application or from dunking in pickle should always be avoided, even with diamond. In recent years, the application of heat to or near a gemstone has become more dangerous with the increased use of "fillers", most of which are some form of glass. Almost every smaller ruby found for sale on the market today is from the Mong Hsu deposits in Myanmar (Burma) and ALL of them are not only heat treated but extensively filled with borosilicate glass. We'll discuss that when we discuss rubies, but glass and heat do not mix well.
Good files have "safe" sides for a reason. Although many common gemstones are much harder than tool steel, facetjunctions or edges can easily be chipped by a file, requiring a trip to the lapidary. If you DO scratch a faceted stone, don't try to "polish out" the scratch. It won't work and you probably don't have the right tools for the job. A simple, inexpensive re-polish can turn into a costly re-cutting job in just a second or two.
I'm going to discuss a variety ofthe colored stones and lapidary materials you are most likely to encounter at the bench. I've chosen an alphabetical convention, using the most common variety names.
Alexandrite is the color-change variety of the mineral chrysoberyl. Generally, the change is from some shade of bluish-green or olive green in daylight to some shade of purplish-red or brownish red in incandescent light. The primary basis to the value of this very expensive stone is the quality of the color change. Most stones (not all) available today are from Brazil and are mostly rather dark, although there are some occasional fine stones from there and from Sri Lanka, SWAfrica, EastAfrica, Russia (rarely) and India.
Currently, fine stones in the l-2 carat range can be found at $2,000 to $6,000 per carat, and fine stones in the 3-5 carat range are seen in the $5,000 to $8,000/c range. Extra fine stones are much higher to $14,000 per carat and that is often for poorly cut goods! The weight loss suffered when cutting to proper proportions and angles is usually considered cost prohibitive, unfortunately.
Alexandrite is quite hard, 8.5 on the Mohs scale and exhibits excellent toughness. While most Brazilian material is very clean, stones from other locations often have many characteristic inclusions. While that is a great help in proving the stone is natural, some of these inclusions can split or even explode on application of heat. Considering the high cost of replacement, it's a wise idea to remove an alexandrite from the setting for all but the slightest of torch work. The pickle should cause no problems; the ultrasonic should not, but avoid sudden heat changes in a hot ultrasonic.
True synthetic alexandrite is available. In gemological terms, the word "synthetic" means that the material has all the chemical and physical attributes of the natural material. It is not an imitation or simulant; it IS the same as the natural material except that it is produced in a laboratory as opposed to having been dug out of the ground. Many dealers and jewelers prefer to use the term "laboratory grown" or "cultured" because they feel the term synthetic is confused with "cheap" by the public. In reality, there are some very inexpensive synthetics and there are some rather expensive "luxury" synthetics, and they all fill a certain demand in the marketplace.
True synthetic alexandrite is one of the more costly synthetics and the degree of color change is dependent on the skill of the cutter and the amount of waste the cutter finds acceptable. This all conspires to drive the price upwards, and good synthetic alexandrite currently trades at wholesale at about $90 per carat to over $200 per carat, depending on size. Independent cutters often charge less, and the cutting is better as well.
Because most synthetic alexandrite is nearly flawless, the material is exceptionally durable and heat resistant. Prolonged direct torch heat is to be avoided, but the pickle and ultrasonic are problem free.
Amethyst is the pinkish-violet to purplish-violet variety of quartz. It can range in saturation from the very light pinkish "Rose de France" to the intense bluish-violet with red overtones of the material from Zambia and Namibia. Most is a medium violet to purple with a slight grayish or brownish modifier, so familiar in the commercial quality stones from Brazil and Bolivia found in large quantities and in every jewelry store in America.
While most amethyst in the 5 to 10 carat range whole- sales for $15-25 per carat in fine quality and $30-45 per carat in extra fine quality, the vast majority of stones seen in jewelry sell for much less. On the other end of the scale, well-cut goods in top colors easily bring $75 per carat and more, and sell very rapidly to design-oriented studio jewelers.
Amethyst is hardness 7 on the Mohs scale, and considered fairly tough. Gemstones softer than amethyst are considered to be on the "soft" side. With normal care, amethyst worn in a ring will gradually show signs of wear at the faceted edges, but can be easily repaired or replaced, except in the highest qualities. It has proven to be very durable in pendants and earrings, and really should give the wearer many years of enjoyment if the ring is not worn continuously or for sport activity.
Heat needs to be avoided when working with amethyst. Even slow application of torch heat is likely to convert an amethyst into a citrine and the change is not reversible. Additional heating will turn the stone colorless. Also, some amethyst has been known to fade when placed in a sunny window for long periods (weeks or months) and exposure to the UV rays of a tanning booth can also fade some amethyst. Sudden quenching can shatter any quartz, but the ultrasonic is generally safe. I suggest warming the stone in water first. The pickle will present no problem.
Synthetic amethyst is readily available and may now account for over 50% of the stones seen today. The cost of natural, fine-colored amethyst is relatively high compared to what can be obtained for finished gems, especially when considering the loss involved of proper cutting, so if you are seeing dark amethyst at low prices, you can bet it's synthetic. The synthetic material is beautiful and durable, and generally avoids the zoning seen in most natural material. Native cut synthetic material trades for under $10 per carat, while custom cut synthetic will range to about $25-30 per carat in 5 carat stones and larger.
The same handling cautions apply to synthetic amethyst as natural amethyst, because the material IS amethyst.
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