OPAL

Opal is the world's most popular phenomenal gem. Many cultures have credited opal with supernatural origins and powers. Arabic legends say it falls from the heavens in flashes of lightning.

The ancient Greeks believed opals gave their owners the gift of prophecy and guarded them from disease. Europeans have long considered the gem a symbol of hope, purity, and truth.

Opal is one of the US birthstones for October (along with tourmaline). Some people think it's unlucky for anyone born in another month to wear an opal, but that particular superstition comes from a novel written in the 1800s (Anne of Geierstein by Sir Walter Scott) and not from ancient belief or experience. In fact, throughout most of history, opal has been regarded as the luckiest and most magical of all gems because it can show all colors. Once, it was thought to have the power to preserve the life and color of blond hair.

Although experts divide gem opals into many different categories, the main types are:

White opal - translucent to semi-translucent with play-of-color against a white or light gray body color.
Black opal - translucent to opaque with play-of-color against a black or other dark body color.
Fire Opal - transparent to translucent with brown, yellow, orange, or red body color. This material, which often does not show play-of-color, is also known as "Mexican opal," "gold opal," or "sun opal."
The market supply of fine black opal is extremely limited, but white and fire opals are generally available in a wide range of sizes. You'll usually see black or white opals fashioned as cabochons and set in rings, pendants, pins, or earrings. Fire opals are used in the same kinds of jewelry, but they're often faceted. All three types occasionally appear as beads and carvings.

Pieces of white or black opal that are too thin to use alone often become part of opal doublets or triplets. In these assembled stones, a sliver of opal is cemented, usually with black adhesive that dramatizes the play-of-color, to a backing such as chalcedony, glass, or plastic. A doublet consists of two pieces (the opal and the backing), while a triplet also has a protective top made of rock crystal quartz or colorless glass.

Explaining Play-of-Color
Play-of-color occurs because opal is made up of sub-microscopic spheres stacked in a grid-like pattern, like layers of ping-pong balls in a box. This structure breaks up light into spectral colors. The colors you see depend on the sizes of the spheres. Those approximately 0.1 micron (one ten-millionth of a meter) in diameter produce violet. Spheres that are about 0.2 microns in size produce red. Those in between produce intermediate hues.

Common trade terms for play-of-color include:

Pinfire or pinpoint - small, close-set patches of color
Harlequin or mosaic - broad, angular, close-set patches of color
Flame - sweeping reddish bands or streaks that shoot across the stone
Peacock - mainly blue and green

Sources



Australia Black and white opal

Brazil White opal

Mexico Fire opal


Hardness & Toughness

Hardness 5 to 6½ on Moh's scale

Toughness Very poor to fair


Stability
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.


Treatments (all treatments listed are common)

Treatment Purpose Stability Detection
Impregnation with oil, wax, or plastic.

Improves play-of-color and prevents or disguises fracturing. Black plastic also creates the appearance of black opal. Fair to poor for oil or wax; excellent for plastic. Oil and wax treatments are detectable by a trained gemologist or gemological laboratory. Advanced laboratory testing is almost always required for plastic. *
Soaking in dye, silver nitrate, or sugar and acid (known as "sugar treatment"). Creates or improves play-of-color and simulates the appearance of black opal. Poor to good Detectable by a trained gemologist or gemological laboratory. *
Smoke impregnation.
Creates or improves play-of-color and simulates the appearance of black opal.

Fair to poor. Treatment is shallow, and abrades or chips away easily. Detectable by a trained gemologist or gemological laboratory. *
* If there is any doubt, send the gem to a gemological laboratory for verification

Care and Cleaning
Opals can be cleaned with warm, soapy water. Avoid ultrasonic and steam cleaning.

Imitations and Synthetics
Glass and plastic have been used to imitate opal, and synthetic opals are available in a variety of colors

Alternatives
No gem duplicates opal's unique combination of color and phenomenon. As alternatives, you might suggest stones with similar body colors, or those that show other special optical effects, such as fire agate, and iris agate.

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EMERALD

Emerald's lush green has soothed souls and excited imaginations since antiquity. Its name comes from the ancient Greek word for green, "smaragdus."


Pliny the Elder described emerald in his Natural History, published in the first century AD: "...nothing greens greener" was his verdict. He described the use of emerald by early lapidaries, who "have no better method of restoring their eyes than by looking at the emerald, its soft, green color comforting and removing their weariness and lassitude." Even today the color green is known to relieve stress and eye strain.

Legends gave emerald the power to make its wearer more intelligent and quick-witted. It was once believed to cure diseases like cholera and malaria. Its color reflects new spring growth, which makes perfect choice of a birthstone for the month of May. It's also the gemstone for twentieth and thirty-fifth wedding anniversaries.

History
The first known emerald mines were in Egypt, dating from at least 330 BC into the 1700s. Cleopatra was known to have a passion for emerald, and used it in her royal adornments.

Emeralds from what is now Colombia were part of the plunder when sixteenth-century Spanish explorers invaded the New World. The Indians had already been using emeralds in their jewelry and religious ceremonies for 500 years. The Spanish, who treasured gold and silver far more than gems, traded emeralds for precious metals. Their trades opened the eyes of European and Asian royalty to emerald's majesty.

Emerald is often mined and sold under peril - the natural resource Colombians cherish is also coveted by underworld drug traders. The availability of fine-quality emerald is limited, and emerald was plagued in the late 1990s by negative publicity about treatments commonly used to improve its clarity.

Gemology
Emerald is the most famous member of the beryl family. It has a hardness of 7.5 to 8 on the mohs hardness scale, and has a toughness of poor to good.

Treatments
Fracture Filling — Filling surface-reaching fractures with colorless oils or resins to improve clarity. (Routine — Not Permanent)

Dyeing — Adding a colorant to the oils or resins used in the fracture filling process to improve color. (Rare — Not Permanent)

Coating — Covering a light-colored beryl with a green plastic to create an emerald imitation. (Rare — Not Permanent)

Sources
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.

Care and Cleaning
Emeralds are routinely treated with colorless oils or resins to improve clarity. To clean emeralds, only warm soapy water should be used. Avoid the use of harsh detergents and vigorous scrubbing.

Simulants, synthetics and alternatives
Emerald imitations can be made from green glass or synthetic spinel triplets.

Synthetic emeralds may be grown using the flux of hydrothermal method.

Emerald alternatives include alexandrite, demantoid garnet, diopside, jadeite, Peridot, sapphire, tourmaline, tsavorite garnet, and zircon.

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ALEXANDRITE

Alexandrite is the color-change variety of the mineral species chrysoberyl, and is the one of the birthstones for June.

The most sought after alexandrites are a lovely green in daylight or fluorescent light, but change to red or slightly purplish red in the incandescent light from a lamp or candle flame.

Abundant alexandrite deposits were first discovered in 1830, in Russia's Ural Mountains. Those first alexandrites were of very fine quality, and displayed vivid hues and dramatic color changes. The gem was named after the young Czar Alexander II, and it caught the country’s attention because its red and green colors mirrored the Imperial Russian flag.

The spectacular Ural Mountain deposits didn’t last forever, and now most alexandrite comes from Sri Lanka, Brazil, India, Tanzania, and Madagascar. The newer deposits contain some fine-quality stones, but many possess less precise color change and muddier hues than the 19th century Russian alexandrites. You may still find some of the famed Ural Mountain alexandrites in estate jewelry. They remain the quality standard for this phenomenal gemstone.

Gemology
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.

Typical size range
Alexandrites typically come in sizes from melee to 5 carats. Larger stones are rare.

Treatments
None known

Sources
Sri Lanka and Brazil are the two major producers. Alexandrite is also found in Madagascar, Tanzania, India, Sri Lanka, and Russia.

Care and Cleaning
Alexandrite can be cleaned in warm, soapy water. Ultrasonic cleaning and steam cleaning are usually safe.

Simulants, synthetics, and alternatives
Synthetic alexandrite was developed in 1973. It can be grown using the flux or Czochralski methods. Synthetic alexandrite will have a similar color change to natural alexandrite, bluish green in daylight and brownish red to purple-red in incandescent light. Synthetic alexandrites often have flux inclusions, triangular metallic platelets, curved striae, or gas bubbles, depending on the growth technique that was used to grow the synthetic. Synthetic alexandrites typically have a slightly lower RI, and stronger fluorescence than natural alexandrites.
Synthetic color-change corundum (of which ruby and sapphire are varieties) was developed approximately in 1909. The color change in alexandrite-like synthetic sapphire will be bluish purple or greenish blue changing to reddish purple, depending on the growth process used to create the synthetics.
Synthetic color-change spinel.

Color-change garnet
Color-change sapphire