Precious Gem Stones Chantaburi Thailand

Buying Gold and Gemstone Jewelry: The Heart of the Matter

If you're planning a gift of gold or gemstone jewelry for your sweetheart, take some time to learn the terms used in the industry. Start with the basics. Carats and karats. What's the difference? Simply put, a carat is a unit of weight for gemstones, including diamonds; a karat is a unit for measuring the purity of gold.

Gold





The higher the karat rating, the higher the proportion of gold in the piece of jewelry. For example, 24 karat (24K) gold has more gold content than 14K gold. But because 24K gold is soft, it's usually mixed with other metals to increase its hardness and durability; 14K jewelry contains 14 parts of gold, mixed in throughout with 10 parts of base metal.

There's a big difference between karat-gold and gold-plated jewelry. Gold-plated describes jewelry with a layer of at least 10K gold bonded to a base metal. Gold plating eventually wears away, depending on how often the item is worn and how thick the plating is.

Gemstones


Gemstones can be naturally mined, laboratory-created or imitation. Stones created in a lab look identical to stones mined from the earth. The big difference is in the cost - laboratory-created stones are less expensive than naturally mined stones. But because they look just like stones mined from the earth, they must be identified as lab-created. Imitation stones resemble naturally mined stones but are not identical and are usually made of glass or plastic. Imitation stones must be identified as imitation. If you are purchasing a naturally mined stone, ask if it has been treated. Gemstone treatments - such as heating, dyeing or bleaching - can improve a stone's appearance or durability. Some treatments are permanent; some may create special care requirements. Treatments also may affect the stone's value.

Jewelry Shopper's Checklist

Once you're ready to buy jewelry, shop around. Compare quality, price and service. If you're not familiar with jewelers in your area, ask family members, friends and co-workers for recommendations. Also, when ordering online and you've never heard of the seller, check on its reputation with the Better Business Bureau or the state attorney general's office. You also should:

• Ask for the store's refund and return policy before you buy. When ordering online, keep printouts of the web pages with details about the transaction, including refund and return policies in case you're not satisfied.
  
  
• Check for the appropriate markings on gold jewelry.
  
  
• Ask whether a gemstone is natural, laboratory-created, or imitation.
  
  
• Ask if the gemstone has been treated. Is the change permanent? Is special care required?
  
  
• Make sure the jeweler writes on the sales receipt any information you rely on when you make your purchase, such as the gemstone's weight or size. Some jewelers also may supply a grading report from a gemological laboratory.

Chemical Formulas of Gemstones

Where Do Gems Form?

Gems form in many different environments in the Earth. We will examine the most common and important environments and formation processes in this lecture.

It is important to distinguish where gems are formed from where they are found.

Almost all gems are formed below the Earth's surface.

• Some are brought to the surface through mining
• Some are brought to the surface through earth processes (faulting, folding, large scale uplift, volcanism). These processes can move rock up from more than 400 km below the surface.

Formation Environments

• Water near Earth's surface
• Hydrothermal deposits
• Pegmatites
• Magmatic gems
• Metamorphic gems
• Gems of the mantle

Water near Earth's surface
Hydrothermal Deposits
Pegmatites
Magmatic Gems

Metamorphic gems

Gems of the Mantle

What is a Gemstone?

Gemstone is a collective term for all objects used with ornamental stones for personal adornment that possesses beauty, durability, and stability. Beauty, a quality that varies among individuals, can be based on one or any combination of: color or lack of color, luster, transparency, or enhanced optical properties due to cutting and fashioning by the lapidary. Durability is the resistance of the gemstone damage and dependent upon physical properties, such as hardness and tenacity. Tenacity is the resistance to bending and breaking, while hardness measures the resistance to scratch. Diamond cannot be scratched by any other mineral and therefore, it is a gem with high hardness; diamond has low tenacity, in part due to perfect cleavage. Nephrite jade is relatively soft for a gem, easily scratched by dust in the air, but it is composed of fibrous crystals that resist breaking and therefore, it is tough or has high tenacity. Stability refers to the gem's ability to retain color inspite of heat, light, or chemical assaults.

Of the 4,000+ known minerals, 70 possess qualifications for gemstones, but of these, only approximately 20 are commonly encountered (Hurlbut & Kammerling, 1991, p. 3). These minerals include: diamond, corundum (ruby, sapphire, star ruby and sapphire), beryl (emerald, aquamarine, morganite, goshenite, golden or heliodor), chrysoberyl (cat's eye & alexandrite), spinel, topaz, zircon, tourmaline (indicolite, rubellite, schorl, elbaite), garnet group (almandite/almandine, rhodolite, pyrope, grossular/tsavorite, spessartine, uvarovite), quartz: crystalline (rock crystal, amethyst, citrine, cairngorm or smoky, rose, aventurine, tiger's-eye, rutilated), quartz: cryptocrystalline (chrysoprase, carnelian, sard, bloodstone, agate, onyx, jasper, agatized or petrified wood), olivine peridot (chrysolite), jadeite jade, tremolite-actinolite or nephrite jade, spodumene (kunzite, hiddenite), feldspar group (microcline amazonite, labradorite, orthoclase moonstone, oligoclase sunstone), turquoise, lapis-lazuli, and opal.

THE COLORS OF GEMSTONES

The most common cause of color in gemstones is the presence of a small amount of a transition metal ion. These transition metal ions have an incomplete set of 3d electrons. Changes in the energy of these electrons correspond to the energy of visible light. When white light passes through a colored gemstone or is reflected by it, some of the energy of the visible light is absorbed, causing 3d electrons in the transition metal ion to undergo an energy change. The light that is transmitted or reflected appears colored, because those colors corresponding to 3d-electron energy transitions have been absorbed. The table lists several common gemstones, their chemical compositions, colors, and the origins of these colors.

Gem	Formula	Color	Origin Of Color
Ruby	Al2O3	Red	Cr3+ replacing Al3+ in octahedral sites
Emerald	Be3Al2(SiO3)6	Green	Cr3+ replacing Al3+ in octahedral site
Alexandrite	Al2BeO4	Red/Green	Cr3+ replacing Al3+ in octahedral site
Garnet	Mg3Al2(SiO4)3	Red	Fe2+ replacing Mg2+ in 8-coordinate site
Peridot	Mg2SiO4	Yellow-green	Fe2+ replacing Mg2+ in 6-coordinate site
Tourmaline	Na3Li3Al6(BO3)3(SiO3)6F4	Pink	Mn2+ replacing Li+ and Al3+ in octahedral site
Turquoise	Al6(PO4)4(OH)8A4H2O	Blue-green	Cu2+ coordinated to 4 OHG and 2 H2O
Sapphire	Al2O3	Blue	Intervalence transition between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites
Aquamarine	Be3Al2(SiO3)6	Blue	Intervalence transition between Fe2+ and Fe3+ replacing Al3+ in adjacent octahedral sites
Diamond	C	Colorless pale blue or yellow Color	centers from nitrogen atoms trapped in crystal

Featured Gemstones

People in every era and from all walks of life have adorned themselves with the dramatic, radiant grace of colored stone jewelry. The world of colored stones is vast and varied, and a true understanding of them requires knowledge of both science and history. Here are just a few of the most popular colored stones today.

Alexandrite

Alexandrite is one variety of the mineral chrysoberyl, named after Czar Alexander II of Russia. It was discovered in 1831, in the Ural Mountains of Russia, and is composed of BeAl₂O₄.

Alexandrite is unusual because it is a grass green in daylight and raspberry red in artificial, incandescent lighting. The trace element chromium is responsible for both the red and green of alexandrite. This same trace element is responsible for the green of emerald and red of ruby when is substitutes for aluminum in the crystal structure. The coloring agents are dependent upon the wavelength of light and the crystal structure/chemical bonding; the chromium in alexandrite is such that the color changes with wavelengths of light, from natural sunlight or fluorescent lighting, where is appears green, to indoor incandescent light, where is appears red.

Alexandrite has a hardness of 8.5 on the Mohs hardness scale, and has excellent toughness. Alexandrites are typically transparent, and may contain fingerprint and silk inclusions.

Emerald

Emerald is the deep green variety of beryl, an aluminum berylium silicate, Al₂Be₃(Si₆O₁₈). It is a cyclosilicate and often found in hexagonal primatic crystal forms. Beryl is 7.5-8 on the hardness scale and a vitreous luster. It has a conchoidal fracture and a brittle tenacity, which makes it sensitive to pressure and heat. Specific gravity is average to medium high, 2.66-2.87, and the refractive indices are 1.562-1.602. Although beryl may have an irregular distribution of color, the color is stable in light and heat. Asterism and cat's eye stones are possible. The best known variety of beryl is emerald, a deep green color. The green coloring agent is the impurity element, chromium, and possibly some vanadium. Some believe chromium defines emerald, whereas beryl colored with vanadium is merely green beryl. The finest emerald are transparent but more commonly they are clouded with inclusions. Dispersion is 0.014 and emerald has distinct pleochroim, showing blue-green and yellowish-green. It has no fluorescence but emerald shows bright red through the color or Chelsea filter.

Colombia is one of the largest commercial producers of emerald. Fine Colombian emeralds are highly regarded for their excellent color. Zambia is also a commercial source of emeralds with good clarity. Other sources include Afghanistan, Brazil, Pakistan, Russia, and Zimbabwe.

Opal

Opal is a hydrated silica, SiO2(H2O), that is lacking in crystal structure or is amorphous. The hardness is 5.5-6.5 and opal has a low specific gravity at 1.98-2.20. It has a conchoidal fracture and is brittle. Opal contains water, from 3-30%, and this over time will be lost, diminishing the opalescence (Schumann, 1997, p. 150). There are basically three types of opal: precious opal (containing flashes of fire), the orangish-red "fire opal" (named for its color, not flashes of fire), and common opal.

High heat or sudden temperature change can cause fracturing. Opals are generally stable to light, but heat from intense light can cause fracturing (known as "crazing"). Opals are attacked by hydrofluoric acid and caustic alkalis. Loss of moisture, and crazing, can result from storage in airtight containers, such as safe deposit boxes.

Pearl and Cultured Pearl

Pearls have been valued for their natural beauty, no faceting or polishing required, for some 6000 years. Approximately 70% are strung and worn as necklaces. Pearl quality is determined according to shape, color, size, and luster. Spherical are the most valued, but many pearls are irregular shapes, which are termed baroque or barrel pearls. Extreme dryness or moisture are both damaging conditions for pearls, as well as acids, perspiration, cosmetics, and hair spray. The low hardness means pearl is easily scratched and therefore storage is an important consideration.

High heat can burn cultured pearls, or cause discoloration, splitting or cracking. Pearls are generally stable to light but heat from intense light can cause dehydration and nacre cracking. Pearls are attacked by many chemicals and all acids. Hair spray, perfume, cosmetics, and even perspiration can damage nacre.

Peridot

Peridot (pronounced pear-a-doe) is the gem variety of the mineral olivine, and also called chrysolite. It is a magnesium iron silicate, (Mg, Fe)2SiO4, with a hardness of 6.5-7, and specific gravity of 3.28-3.48. Cleavage is indistinct and the fracture, conchoidal. Crystal form is rare, although it is in the orthorhombic crystal system. It is doubly refractive with refractive indices of 1.63 and 1.695 ). Pleochroism is weak and peridot has no fluorescence.

Peridot stability is Rapid or uneven heat can cause fracturing in peridot. The gem is stable to light, but is attacked easily by sulfuric acid, and less easily by hydrochloric acid. Long-term exposure of perspiration may also attack peridot.

Ruby

Ruby occurs in metamorphic rocks, dolomitic marble and gneiss, whereas sapphire may be found in sedimentary limestones, metamorphic marble, and igneous basalt, pegmatites, or andesite dikes. Corundum is very rarely mined from primary deposits, but typically mined from secondary alluvial or placer deposits, where this resistant mineral becomes the sand and gravel of stream beds. Ruby and sapphire have been synthesized since the end of the 19th century for industrial and gem quality uses. Synthetic star ruby and sapphire have been marketed since 1947.

Heat can cause a change in color or clarity, it can also damage or destroy fracture- and cavity-fillings. Rubies are generally stable to light, but bright lights can cause oil to leak or dry out. Chemicals can harm fillings and remove oil; soldering flux containing boron and firecoat made with boric acid powder, will etch the surface of even untreated stones.

Spinel

Spinel is a good candidate for the title of “History’s Most Under-Appreciated Gem.” Some ancient mines that supplied gems for royal courts from Rome to China produced spinels, but they were usually confused with better-known stones like ruby and sapphire.

Spinel’s color range includes violet, blue, orange, red, pink, and purple. Blue spinels are often grayish and subdued, but the best are a deep rich color. The reds can rival fine ruby. And the vivid orange to orange-red stones merit their trade name—flame spinel. Some spinels show color-change, usually turning from grayish blue in daylight or fluorescent light to purple under incandes­cent light.

Tanzanite

Tanzanite is a member of the zoisite group, as is thulite, the national gemstone of Norway. Thulite is an opaque, pink gemstone usually cut en cabochon, and is named after Thule, an old name for Norway. The gem quality transparent zoisite, tanzanite, varies from a deep "sapphire" blue to an "amethyst" purple. It is now one of the birthstones accepted for December birthdays and is a calcium aluminum silicate, Ca2Al3(O/OH/SiO4/Si2O7).

Tanzanite is special-care gem for two reasons: sensitivity to thermal shock and the potential for cleavage. Sometimes the temperature change between the hot lights of the display case and the chilly glass countertop in an air-conditioned showroom can be enough to develop cleavages in tanzanite.

Topaz

Topaz is a nesosilicate, Al2[(F,OH)2/SiO4], and 8 on the hardness scale. It is in the orthorhombic crystal system, which is reflected in the commonly found prismatic crystal form with orthorhombic pyramid terminations. It has perfect basal cleavage, as well as a conchoidal and uneven fracture. Topaz is transparent and translucent, with a vitreous luster, and high specific gravity, 3.49-3.57. It is doubly refractive, 1.609-1.643, with a strong to definite pleochroism (with the exception of blue topaz, which as weak to none), and weak fluorescence. When topaz is gently heated or rubbed, it becomes electrically charged. Topaz occurs in cavities within granite or rhyolite igneous rock, pegmatite dikes, high temperature quartz veins, and/or alluvial deposits.

The largest faceted gemstone ever recorded is the "Champagne Topaz." The gem was mined in Brazil, weighs 36,854 cts. and measures 34.9 cm x 15.5 cm x 12.1 cm.

Turquoise

Turquoise is hydrous copper aluminum phosphate, CuAl6[(OH)2/PO4]4 4H2O (Schumann, 1997, p. 170). It varies in color from a sky blue to blue-green or apple-green, and is frequently fashioned with the matrix rock and interspersed vein rock. The veined material is called "spiderweb" turquoise. The blue color changes to green with high heat (of soldering) and exposure to light, perspiration, oils, cosmetics, household detergents (always remove turquoise rings before washing hands or dishes). It has a hardness of 5-6, cleavage, and a conchoidal fracture. The specific gravity is 2.31-2.84 and it is in the triclinic crystal system, crystals are rare. It has a waxy luster and both the fluorescence and pleochroism is weak.

Historically, the finest material was mined in Iran and is known as Persian turquoise. This source is no longer commercially important. Today, the United States is the major source of turquoise. Other sources are China, Chile, Australia, and Mexico.