Tuesday, 31 December 2013

Treasures of the British Museum I

Cross pendant. Gold with a closed back and set with a central pearl surmounted by a garnet and an emerald, flanked by hessonite garnets, a sapphire, and a zircon with a ruby, a chrysoberyl cat's eye, an amethyst and a zircon below. Date 1800 (circa)

Gimmel-ring; gold; enamelled; bezel, set with ruby and emerald, in form of quatrefoil flower with pendant leaves decorated with blue, black and white scrolls; inner faces of bezel decorated with scrolls; shoulders moulded in form of scrolls; inscription on inner surfaces revealed when ring opened. Date 16thC, Germany.
Pendant in the form of an animal head. Made of wood and covered with turquoise and malachite mosaic held in place with pine resin adhesive. The eyes are made from pyrite and white and yellow striped conch (Strombus) shell. The open mouth is encrusted with gemstones (garnet, beryl, emerald, spinel, zircon) and lined with sharks teeth. The pearls, gemstones and teeth are all held in place with beeswax. Culture/period Mixtec; Aztec

Two-colour gold comb-mount in the form of a leafy twig surmounted by a bird, with a ruby eye and ring in its beak, on a trembler spring. The branch is set with gemstones whose initials spell 'dearest', ie. diamond, emerald, amethyst, ruby, emerald, sapphire and turquoise. There is a hair compartment in the reverse of the bird. Date 1830 (circa) England.
Intaglio of rock crystal: massive, oval, convex on one side, flat on the other. On the flat surface is engraved the Crucifixion between the Virgin and St John; Christ has the cruciferous nimbus and wears a loin-cloth, his head is inclined; cross is plain, without titulus, serpent coiled round its foot; Virgin and St John stand bending forward, raising their mantles to their faces in attitudes of grief; above cross are two medallions containing busts of the sun and moon holding torches, former radiate and wearing chlamys, latter as female figure in mantle with head surmounted by crescent; in modern metal mount; each side of crystal with cylindrical drill-hole. Culture/period : Carolingian Date 846-869 (?)

Gold memento-mori fede-ring. This large ring, either intended as a thumb-ring or to be worn over a glove on the finger, is made of gold, enamelled and set with gemstones. The bezel is in the form of a book, placed horizontally, the upper cover being set with four table-cut gemstones in plain gold collets: a diamond (lower right), a ruby (upper right), an emerald (lower left) and a sapphire, or blue spinel (upper left); in high relief between the gemstones, in the centre, is a white-enamelled skull with a green-enamelled toad above and another below, and two snakes issuing from the skull to left and right. Date 1526-1575

Saturday, 28 December 2013

Synthetic Diamonds

The closest synthetic approximation to diamond is a man-made diamond. Man-made diamonds can be made of pure carbon.
The Gemological Institute of America (GIA) recognizes these as real diamonds from a compositional perspective. But, the man-made diamonds don't have the rich geological history that natural diamonds do. Laboratories simulate the heat and pressure from the Earth's mantle that create natural diamonds. For some, diamonds come down to a matter of time and money: days versus millions of years, thousands of dollars versus tens of thousands of dollars or more.

If a uniquely coloured, relatively inexpensive diamond is desired, man-made ones in shades of orange, yellow, pink and blue are readily available. Finding a large diamond will prove a greater challenge -- most man-made diamonds weigh less than one carat. To prevent retailers from passing off man-made diamonds as natural ones, the GIA is selling machines that will help jewelers easily distinguish between the two.

It may come as no surprise that the developer behind these machines is none other than the king of the natural diamond industry: De Beers.

Wednesday, 25 December 2013

Ceylon Sapphires

For centuries, the island of Sri Lanka has been a key source for a variety of gemstones, including sapphires. Sri Lankan gems were mined, set into jewelry, and traded abroad since at least 500 B.C. The Etruscans, Greeks, and Romans coveted sapphires, and the Indians called Sri Lanka “Ratna deepa,” which means “Island of Gems.”
Many consider Sri Lankan sapphires to be among the best in the world.

A range of colored sapphires can be found there, including rare padparadschas. Approximately 90 percent of the world’s star sapphires also come from the island. The sapphires from Sri Lanka are known for their high clarity, rutile silk, and fingerprint inclusions. Current sapphire yields are estimated to be 60 percent blue, 25 percent yellow or orange, and 15 percent pink or purple.
Sri Lanka’s sapphires come from extensive gravel deposits located in the southern two-thirds of the island. Although the original source of this gravel remains unknown, scientists speculate that the parent rock is a Precambrian metamorphic rock that makes up about 90 percent of the island.

Experts speculate that the erosion of this parent rock has created the extensive gem deposits along ancient and current riverbeds in the lower valleys.

Mining methods are relatively primitive and when gravel is extracted from current riverbeds, it is done with hand-made scrapers.
On land, miners typically use simple non-mechanized equipment, including picks, shovels, spades, and baskets. When shafts are sunk to reach the gravel, they are reinforced with palm and bamboo scaffolding and pumped to keep the water level down. At some locations, bulldozers scrape the overburden, but the gem gravel is still washed by hand.

Marco Polo wrote that the island had the best sapphires, topazes, amethysts, and other gems in the world.

Ptolemy, the 2nd century astronomer recorded that beryl and sapphire were the mainstay of Sri Lanka’s gem industry.
Geologically speaking Sri Lanka is an extremely old country. Ninety percent of the rocks of the island are of Precambrian age, 560 million to 2,400 million years ago.
The gems form in sedimentary residual gem deposits, eluvial deposits, metamorphic deposits, skarn and calcium-rich rocks. Other gems are of magmatic origin.

Residual deposits are mainly found in the flood plains of rivers and streams. Metamorphic types of gems constitute 90% of the gem deposits in Sri Lanka. Blue sapphires from Sri Lanka are known as Ceylon Sapphire. Ceylon Sapphires are unique in colour, clarity and lustre compared to the blue sapphires from anywhere else.

Tuesday, 24 December 2013

Star Sapphire

A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism; red stones are known as "star rubies". Star sapphires contain intersecting needle-like inclusions following the underlying crystal structure that cause the appearance of a six-rayed "star"-shaped pattern when viewed with a single overhead light source. The inclusion is often the mineral rutile, a mineral composed primarily of titanium dioxide.

The value of a star sapphire depends not only on the weight of the stone, but also the body color, visibility, and intensity of the asterism.
Star Sapphire is usually found in blue colors, but there are also various shades of brown and green that are called black star sapphire. Orange and yellow star sapphires are almost unknown, and very rare. Color changing star sapphires are even more of a rarity.

The coloring agents in blue sapphire are iron and titanium and, in violet stones, vanadium. A small iron content only results in yellow and green tones, chromium produces pink, iron and vanadium orange tones.
The most desirable color is a vivid, intense blue.

Less transparent sapphires, translucent or opaque stones, are cut en cabochon to support the star effect with its six rays. The best cabochons are somewhat transparent, with smooth domes of good symmetry.
The Star of India is a 563.35-carat star sapphire, one of the largest such gems in the world. It is almost flawless and is unusual in that it has stars on both sides of the stone. The greyish blue gem was mined in Sri Lanka and is housed in the American Museum of Natural History in New York City.
The Black Star of Queensland is a 733-carat black sapphire, and the world's largest gem quality star sapphire. It was discovered in Australia in the 1930s.