Examples of rocks from the 5 major metamorphic facies from the previous chapter. The plate-tectonics figure shows the geologic setting in which most rocks of these different groups form.
Gems from metamorphic rocks Gems from metamorphic rocks include popular faceted crystal varieties: corundum including ruby and sapphires , beryls including emeralds , garnet , spinel, and zircon. Non-faceted gem minerals one that are usually cut and polished into cabachons or shapes include jade jadeite ,and quartz varieties jasper, carnelian and tiger eye , and copper-based gemstones including lapis lazuli and turquoise.
Other metamorphic minerals used as gemstones include andalucite , epidote , enstatite , kyanite , and staurolite. Although gem-quality diamond also form under the extreme heat and pressure conditions in the upper mantle depths, they only find their way to the surface in unusual igneous processes associated with kimberlites and related igneous rocks. Corundum varieties include Ruby and Sapphire. Corundum is a very hard mineral of aluminum oxide Al2O3 that occurs in fine- to coarse-grained massive and crystalline forms.
Gemstone varieties are ruby red and sapphires commonly blue, but include yellow and green varieties Figures and Beryl includes gem varieties include emerald, aquamarine, heliodor, and morganite, however only some of these form in metamorphic setting Figure Emeralds are a classic example of a gemstone usually found in metamorphosed shales due to regional metamorphism or quite separately in veins of hydrothermal origin in association with contact metamorphism Figure Because metamorphism takes place in a solid state, inclusions of other solids are usually trapped as the emerald grows around them.
Emerald having inclusions of other minerals is so typical, that any flawless emerald is immediately suspected as a synthetic or even a fake! The inclusions in an emerald are referred to as its "jardin" French for Garden. In many areas, natural weathering and erosion have liberated the stones from their host rock, and carried them into streams over long periods of geologic time.
Today, the gems are mined from these stream sediments. Their high specific gravity relative to other sediment particles often causes currents to concentrate them in small placer deposits. Most rubies and sapphires are produced by washing the gravels of these stream deposits.
This work is often done by hand because the deposits are small and irregular in shape and character. These deposits are often located in countries when wages are very low and artisanal mining is prevalent. Black star sapphire: A black star sapphire 8 mm x 6 mm cabochon from Thailand. Inclusions within the stone align with the crystallographic axis to produce a six-ray silvery star. When the star is clearly visible and centered, as in this example, the base of the stone intersects the c-axis of the corundum crystal at 90 degrees.
This stone has been heat treated to darken the stone and enhance the visibility of the star. Two of the more spectacular events in the history of gemstone mining occurred when heat treatment discoveries enabled geuda a milky white to brownish corundum, found mainly in Sri Lanka to be converted into beautiful blue gems. Worthless corundum had suddenly become valuable!
Until then the worldwide resource of blue sapphire rough was becoming more limited with each passing year. This discovery produced an instantaneous increase in the sapphire resource of Sri Lanka, and possibly similar increases in the sapphire resource in other parts of the world. Within a short time, similar heat treatment methods were being used on a variety of smoky corundum called "dhun" that is found in Madagascar. It was easy to treat and extremely abundant, with no other competing use.
This produced another increase in the world sapphire resource through the discovery of a treatment method. Then came a treatment known as "lattice diffusion. Heat causes the corundum lattice to expand enough that the tiny beryllium atoms can enter. When the corundum is cooled, the lattice begins to contract to its original size and shape, but the trapped atoms inside prevent that.
The deformed lattice then transmits light in a different way, and the color of the corundum is changed. Beryllium diffusion can produce orange, yellow, and pink colors. Titanium diffusion can produce blue corundum. These heat and lattice diffusion treatments have transformed worthless material and worthless deposits into valuable resources.
They enabled an additional income stream from working mines, and suddenly gave previously-mined sediment in many parts of the world another chance at production.
The value of this rough will not be as high as the value of rough with a natural blue color, but it will mean future jobs, future gems, and future sales.
Detecting Synthetic Corundum: Examination with a microscope is the best method for detecting synthetic ruby and sapphire. When these gems are manufactured, growth features and other characteristics provide some of the strongest evidence for the synthetic manufacturing of ruby and other varieties of corundum. In the flame fusion synthesis method, growth lines develop in the crystal as the boule turns under the material feed. Near the center of the boule, these growth lines have a strong curvature.
Near the outer circumference of the boule, the growth lines have a much gentler curvature. The growth lines can be difficult to see. They are only visible when viewed at a limited range of angles under certain lighting conditions.
The growth lines in this synthetic ruby are very coarse. Their crossing of facet junctions confirms that they are within the stone and are not polishing lines on facet surfaces. Synthetic star ruby: Laboratories have been able to mass-produce synthetic star corundum since the Lindy division of Union Carbide flooded the gem market with them in the s and s.
This synthetic red corundum has a visible six-ray star and a faceted back to enhance the brightness of the stone. Rubies and sapphires have been highly sought after in many parts of the world for over one thousand years. Deposits that produce high-quality stones of good color have attracted enormous amounts of attention and have been heavily exploited. As a result, buyers who need large quantities of quality stones are having a harder time finding them in the volumes needed for today's jewelry marketplace.
Let's imagine a jewelry manufacturer who wants to create enough matching ruby pendant, ring, and earring sets to supply a large jewelry chain with over stores and a busy internet site. This manufacturer will need at least four nice rubies for each matching set, multiplied by enough sets to supply over stores and a busy internet site. This manufacturer will need hundreds of thousands, if not millions of rubies, all color-matched into sets and all cut into calibrated shapes and sizes.
On the production side, the labor needed to discover, mine, grade, cut, and polish these stones will be enormous. An enormous effort will also be needed on the manufacturing side just to find enough sellers to provide them, confirm their quality, negotiate prices, make large numbers of purchases, and deliver the stones to the manufacturing facility.
Sourcing millions of natural rubies, sorted and cut to specifications will be a very difficult and time-consuming job. However, sourcing synthetic stones is a much easier and less costly job. This is why laboratories capable of reliably producing synthetic rubies and sapphires of consistent size, color, grade, and appearance have found an important place in the gemstone market.
The synthetic corundum in this jewelry is perfect in color, has wonderful clarity and is extremely attractive. Many shoppers see the lower price and better appearance of the synthetic materials when compared to natural stones of similar size and opt to purchase the synthetic.
It is a logical choice based upon what appeals to the person and what they are willing to pay. They get great appearance at a lower price. Natural gemstones are a finite resource that will become more difficult to obtain and more expensive over time. As a result, buyers will probably see more synthetic stones offered in most jewelry stores and should expect to see the price difference between synthetic stones and natural stones of similar size, color, and quality become greater in the future.
Trying to Fool Gemologists? You are looking down through a ruby that is broken by a honeycomb-shaped network of fractures. The diagonal and slightly curved striations are strong evidence that this ruby is synthetic. It is possible that this synthetic ruby was quench crackled to ruin its perfect clarity and make it look more like a natural ruby - both to the naked eye and through a microscope.
Synthetic rubies, sapphires and other types of gems are easy to find in the marketplace. Many stores sell them, and they account for a very significant percentage of the rubies and sapphires sold today. There is nothing wrong with selling them and nothing wrong with buying them. However, the essential part of the transaction is for the seller to clearly disclose that they are man-made and for the buyer to fully understand that they are man-made.
Sellers should make this essential communication at the time of sale by displaying synthetic gems with obvious labels, informing the customer verbally, and providing a receipt that clearly indicates that they are man-made. They might be called "man-made," "synthetic," "lab-grown," "lab-created" or some other terminology that the buyer understands.
A benefit of buying a natural ruby or a natural sapphire is knowing that your gem was created by nature. A benefit of buying a synthetic ruby or sapphire is obtaining a stone with excellent clarity and color at an affordable price.
Many people have additional benefits in mind when they go shopping for jewelry. You should know that various types of rubies and sapphires are offered for sale. Some are natural gemstones, some are natural gemstones that have been treated by people to improve their appearance, and some are synthetic gemstones created by people.
Many buyers have a strong preference for untreated natural gemstones and are willing to pay a premium price for them. There are several such fee locality sapphire mines in Montana that have been profitable for both the mine owners and the prospectors who have chosen to pay the fee.
The prospector pays a set fee for the right to extract a given weight or volume of sapphire bearing rock or gravel. The fee locality operators allow only the use of hand tools such as picks, shovels, crowbars, gads and wedges. The miner extracts the sapphire bearing rock with the larger tools.
This material is sieved through a series of screens that allow the miner to separate and discard the extremely large cobbles as well as the very fine grained sand, silt and clay. The volume the miner pays for is actually only that that is in the size range that will produce the most useable stones.
The sieved residue is placed in buckets and toward the end of the day the miners return to the base camp where the operators have electric shake tables that wash the gravel and concentrate the particles with high specific gravity toward the bottom of a pan. A half-ton of gravel may actually yield only a few pounds of sapphire bearing concentrate. When the concentrate is collected in the pan, the pan is removed from the shaker and is dumped over quickly on a flat table. The largest sapphires will normally be on the top of the pile or what was earlier the bottom of the pan.
Smaller stones may be found through the rest of the concentrate. In addition to sapphire being found in Montana, some stones have also been produced from mines in North Carolina where several fee localities exist. Alluvial gravel that covers much of the North American mid-continent has also produced an occasional sapphire for the lucky prospector.
Many of the colorless stones from Sri Lanka can be enhanced by heat treatment. There are many other localities of lesser importance throughout the world that have produced sapphires. Johnson and Coeval , p. Southland and M. Garland stated that corundum formed in two distinct geochemical environments. Sapphire form in rocks that geologists call undersaturated, or rocks that have no free quartz. Spinel is another gem that forms in undersaturated rocks.
Spinel may come in many of the same colors as sapphire and in earlier times many stones that were called sapphire or ruby included both corundum and spinel.
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