Gemstone Basics

Precious vs. Semi-Precious Gemstones
The difference between precious and semi-precious gemstones is rarity. Diamonds, rubies, sapphires and emeralds are very rare and are therefore considered to be precious stones; all other gemstones are considered semi-precious

Natural vs. Lab-created Gemstones
Natural gemstones are found in nature, created in and mined from the earth. Natural gemstones are sometimes enhanced, which means they were treated in some way (such as heat) to improve their color and/or clarity.
Lab-created gemstones are made in a laboratories and are chemically, physically and optically identical to gems found in nature. Lab-created gemstones simply don't have the rarity or value of natural gemstones.

Color, Cut and Clarity

• Color - sometimes referred to as the gem's hue. Generally, gemstones with clear, medium-tone, intense and saturated primary colors are most preferred.
• Clarity - appearance of inclusions in the gem. Some gems, such as emeralds, are more likely to have inclusions, while gems like aquamarine and topaz generally have very few or no inclusions.
• Carat - weight of the gemstone. It is important to recognize that some gems are denser than others, so similarly sized stones of different varieties may differ greatly in cost. For example, a one carat ruby (very dense) is going to be smaller than a one carat emerald (less dense).

Gemology a brief introdruction

Gemology is the study of gemstones. Some dictionaries define it as the “scientific study of gemstones; it is almost impossible to remove the scientific element. There may be investors whose only interest is in the value of the stones, but if they ever need to distinguish one gem from another, they are dealing with science.
Also it can be described as bellow,
Gemology is the science dealing with natural and artificial gems and gemstones.It is considered a geoscience and a branch of mineralogy. Some jewelers are academically trained gemologists and are qualified to identify and evaluate gems.

There are many categories of gemologists. Jewelers need to understand gemology in order to be able to answer their customer’s’ questions and identify the gems brought to them.

The goldsmith needs specific knowledge about the physical characteristics of gems. A setting that would be ideal for a diamond would be inappropriate for an opal and vice versa. The amount of pressure used to set the prongs on a garnet would break a tanzanite.

Gem Deposits Of Sri Lanka

Classification of the gem deposits of Sri Lanka

Sedimentary gem deposits

Sedimentary gem deposits are by far the most important of all gem deposits in Sri Lanka and were classified by Dahanayake et al. (1980) into residual, eluvial and alluvial types. The sedimentary placer gem deposits occur in thin layers or lenses of gravel and sand, termed locally as Illam, in river beds and alluvial plains and on hillslopes and hillsides. Among the most important factors that govern the depositional nature of these gem deposits are the intensity and distance of transportation from the source and the topographical suitability of the sites for deposition.

The residual gem deposits occur as beds containing gem minerals mostly deposited in-situ and are found at depths ranging from a few centimeters to about 10 meters. These deposits mostly occur on the flood plains of rivers and streams and their sources are assumed to be in the close vicinity. A characteristic feature of the residual gem deposits is the presence of layers of alternating sand, clays and laterites containing angular fragments, as exemplified by the Elahera gem deposit.

The eluvial sedimentary gem deposits are found on hillslopes and flat areas incised by valleys. Often, the eluvial deposits grade into alluvial deposits making identification difficult. The presence of rock fragments and the angular to sub-rounded nature of the gem minerals are characteristic of the eluvial beds.

Alluvial gem deposits are the most widely distributed gem deposit type in Sri Lanka, the Ratnapura gem deposits being a good example. They often reach depths of more than 20 metres and usually contain two or three gem bearing layers. They occur mostly in old stream terraces and flood plains and are characterized by well-rounded grains. The gem-bearing layers in these alluvial deposits are markedly heterogeneous exhibiting a variety of shapes and sizes that indicate frequent changes in stream courses and velocity.

Skarn and calcium-rich rock types

Recent research has shown that calcium-rich bed- rock is a source for gem minerals within the metamorphic terrain of Sri Lanka. The mineral and fluid inclusions in gem corundum from Sri Lanka and noted that all the fluid inclusions are pure CO2. Thus CO2 is an important indicator of the genesis of the gem minerals. The microthermometry results for the primary inclusions suggested that these corundum formed under granulite facies metamorphism (> 630 ̊C, 5.5 kbar), while the presence of secondary fluid inclusions indicated retrograde post metamorphic cooling and uplift of the source areas. Further, the high density of the fluid inclusions (average density d = 1.05 g/cm3) was considered as being compatible with the formation of corundum under granulite facies metamorphism. Gem deposits located at Bakamuna near the main Elahera gem field an example of a corundum-bearing skarn deposit, and some other examples of this type, at Elahera and at Ohiya respectively.

Metamorphic gem deposits

Most of the gem deposits in Sri Lanka are clearly of metamorphic genesis reflecting the fact that approximately 90% of Sri Lanka comprises high grade metamorphic rocks. Intense tropical weathering has decomposed and disintegrated the gem bearing rocks to form sedimentary gem deposits.

Gems of pegmatitic origin

Pegmatites are common in the Highland Complex and  they  are  also  considered  as  important sources of gem minerals. One of the best known pegmatitic gem deposits is the moonstone deposit at  Meetiyagoda,  southern  Sri  Lanka . In addition, moonstones have been located in regions around Balangoda and Kundasale near Kandy. Pegmatites in Sri Lanka also contain gem minerals such as beryl, chrysoberyl, zircon and corundum.

Aluminous metasedimentary rock types

One of the characteristic features of the Highland Complex is the abundance of aluminous metasedimentary rocks. These have the chemical composition required for the formation of corundum and other aluminous gem minerals. It is clear that in the Highland Complex a combination of the P-T conditions and a suitable chemical composition has yielded voluminous sources for gems of this type. It is known as these gemstones have an origin related to granulite facies metamorphism involving CO2 flooding, the purging of H2O-rich fluids and partial melting.

Basics of Gems

Properties of Gems

The qualities sought in gems are beauty, rarity, and durability. The beauty of a gem depends primarily on its optical properties, which impart its luster, fire, and color; the durability depends on hardness and resistance to cleavage or fracture. The physical properties by which gems are distinguished from each other are form of the crystal, index of refraction of light, hardness, presence or absence of cleavage, type of fracture (conchoidal, even, or uneven) in stones without cleavage, specific gravity, color, streak (color of the powder as determined by rubbing it over white, unglazed porcelain), luster (appearance of the surface in reflected light—adamantine, vitreous, resinous, greasy, silky, or pearly), and transparency. Minor properties that serve to identify some stones are chatoyancy (changeable luster or color under undulating light), opalescence, asterism (starlike sparkling), play of color, fluorescence, phosphorescence, iridescence, and electrical properties. The unit of weight used for gemstones is the metric carat; one carat equals 200 mg.

Gem Cutting

Gems are generally cut to bring out their natural color and brilliancy and to remove flaws. In the cabochon cut, the upper surface of the stone is smoothed and rounded into a simple curve of any degree of convexity; the lower surface may be concave, convex, or flat. All the remaining cuts have flat facets. In the table cut, the facets of the natural octahedron of the diamond are ground to smoothness and polished; one facet, the table, is ground much larger than any other and made the top of the gem, while the opposite facet, the culet, is left quite small. The rose cut consists of a flat base and (usually) 24 triangular facets—resembling a cabochon with facets. The brilliant cut is scientifically designed to bring out the maximum brilliancy of the stone. The crown of a brilliant consists of a table and 32 smaller facets, of which 8 are quadrilaterals and 24 are triangles; the base, of a culet and 24 larger facets, of which 8 are quadrilaterals and 16 are triangles. The base and crown are separated by a girdle. The brilliant cut has certain proportions—in general, the depth of the crown is one third the depth of the stone and the width of the table one half the width of the stone. The trap, step, or emerald cut consists of a table and quadriangular facets above and below the girdle with parallel horizontal edges. Diamond cutting and the cutting of other precious stones are distinct trades.

Diamond Cutting

In diamond cutting the stone is first cleaved or sawed to remove excrescences (outcroppings) or to break it into smaller stones. Cleaving is accomplished by making a groove in the surface in the direction of the grain, inserting a steel knife, and striking the back of the knife a sharp blow. The next process was formerly bruting, i.e., roughly shaping two stones by rubbing them against one another. In modern practice the stones are sawed with a revolving wheel coated on its rim with diamond powder, then shaped by inserting a holder, or dop, containing one diamond into a turning lathe that revolves it against a stationary diamond. The cutting of the facets and the polishing are done by a revolving iron wheel charged with diamond dust. After the facets are cut, the diamonds are cleaned and are ready for sale. Other Gemstones The cutter of gemstones other than diamonds is known as a lapidary. Precious and semiprecious stones other than diamonds are cleaved or slit by a revolving diamond-dusted wheel, faceted by being pressed against a lap (a smoothing and polishing tool) charged with diamond dust or a carborundum wheel, and polished with a softer abrasive. Most (and in the case of some gems all) of the work of faceting is done with only the eye of the lapidary as guide.

Gems and Diamonds

It’s amazing how many different types of gems there are! First of all, you should know that specific types of gems are associated with specific rock types. When you know about rocks, it’s easier to understand why different types of gems are common or extremely rare. For example, quartz is a common item, making up most of the sands on a beach. However, gem quartz only occurs in certain, limited environments.
The most important sources of the world’s colored gems are in sedimentary deposits, such as gravel beds. Marbles and some other types of gems are found in metamorphic rocks. Because most gem mines are smaller than diamond mines, and even most of those are only worked when demands are high enough, a “flood” of colored gems is prevented from entering the world market all at once. There are over 100 types of gems to choose from, and it’s a daunting task to search through the many different types, colors, and shapes that are available! One thing that helps when you’re looking through types of gems in order to purchase a gift, is to have a color of gem and budget in mind.Birthstones are a good place to start! These types of gems come with a direct connection to the person you’re shopping for, and are beloved by many gem enthusiasts.

Once you start looking, it’s amazing how many types of gems there actually are! The gemstone market is more or less based on demand and rarity. You will always pay more for types of gems that are valued and that are hard to find. If you are attracted to a certain color gemstone, you can usually find a synthetic or imitation jewel that glimmers just as enticingly as the real thing. But if you are looking for real jewelry with the label “natural” or “genuine” then you should know a little bit about the origins and terminology of types of gems.
Firstly, you should know that certain types of gems are associated with certain rock types. Once you are familiar with rocks, it will be easier to understand why different types of gems are common or extremely rare. For example, quartz is quite easy and common to find in nature because it makes up most of the sand of the world’s beaches. However, gem quartz only occurs in specific and limited environments, thus it is far more costly and valued.
The most significant sources for types of gems are in sedimentary deposits, such as gravel beds. Marbles and a few other types of gems are found in metamorphic rocks. All types of colored gems are kept at a relatively high value and rarity on the market, because gem mines are only worked when demands are high enough. This successfully prevents a flood of types of gems from entering the world market at one time.It can be a daunting task to find the right choice among all the types of gems available! Here are some tips to help your search:
What color do you have in mind? It can narrow your search if you are looking for a specific color, not a specific type.

Have a budget in mind of what you are willing to spend.
Only purchase “natural” types of gems from a jeweler that you trust. This label technically means that the gem has not undergone any type of treatment. It has only been polished and cut from the time it passed from nature’s hands to man’s. Thus, you will pay a high price for truly natural gems, and you won’t want to be scammed!
If you do want a natural gemstone, the most common types of gems that aren’t treated are garnets, peridot, hermatite, alexandrite and moonstone.
Always compare prices of jewelry with other stores and ads.
Ask for the details on every component of the types of gems you are interested in.
Good synthetic types of gems cannot always be considered cheap when it comes to your budget, but they will be far less costly than natural stones of similar quality.



Thanks to the original publishers of this content.