Stars among gems

Posted on December 5, 2013 by Duncan Parker
Star Sapphire at Dupuis Auctioneers

Star Sapphire at Dupuis Auctioneers

Stars are rare among gems, but given the right conditions, the right crystallographic structure, the right gem, and the right inclusions, all working together, a gem might exhibit a star. These four star sapphires are shown in a group, a constellation, and were all offered in the November 2013 Dupuis fine jewellery auction, lots 183 (go to: http://bit.ly/18orDoj ), 282 (go to: http://bit.ly/IGsQAH ), 454 (go to: http://bit.ly/1eTAHWu ), and 655 ( go to: http://bit.ly/1kgeza4 ) in the sale. The catalogue can still be viewed at www.dupuis.ca

Star Sapphire at Dupuis Auctioneers

Star Sapphire at Dupuis Auctioneers

Sapphires will usually show a six rayed star, some gems (garnets and spinel, diopside) might show a four rayed star, while others may show as many as twelve rays.

The star in a sapphire is caused by light reflecting from the surface of parallel needle-like inclusions of a mineral called rutile, formed in relation to the hexagonally related growth structure of the sapphire.
The rays of the star are perpendicular to the direction of the inclusions (think of light reflecting from combed hair pulled into a pony tail, or reflecting from a spool of thread: The line of reflection is in the opposite direction to the direction the hair or threads are going). The star will only show on a domed surface such as these cabochon cut gems, or beads or spheres, and not on a flat faceted surface.

Star Sapphire at Dupuis Auctioneers

Star Sapphire at Dupuis Auctioneers

What makes a star gem desirable is a combination of the features that make any gem desirable: Colour, clarity, carat weight, and cut, plus it has the added bonus of the phenomenon of a star. The best star will be well-centred, very distinct, have all of the “legs” of equal strength, and as straight as possible.
In auction catalogues, star gems are often shown without the star in evidence. This is because the sharp, single source, un-diffused light required to show the star will usually detract from the photograph in most other ways, and the colour of the gem doesn’t show as well as it can.

Sometimes a star sapphire may also have the added bonus of having a colour change, such as lot 244 in the November Dupuis sale. This sapphire has a six rayed star, plus it has a colour that is purple in candlelight or incandescent light, and blue in daylight or fluorescent light (this gem was mentioned in an earlier post too).

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Pleochroism in tanzanite

Posted on December 1, 2013 by Duncan Parker
Lot 332 from the recent Dupuis auction (dupuis.ca) is a 12 carat tanzanite that exhibits fine colour.

Lot 332 from the recent Dupuis auction (dupuis.ca) is a 12 carat tanzanite that exhibits fine colour.

Tanzanite is a gem noted for its bright colour, its high degree of clarity, and its almost unique source. Tanzanite is an unusual gem because it is found near Arusha, Tanzania, and really not in any commercial quantities anywhere else. The gem was popularized in 1967 by Tiffany & Co (tiffany.com).

Another lot from the same auction, also at dupuis.ca, lot 376, is an oval of 9.29 carats and has a similarly fine colour, but a different shape.

Another lot from the same auction, also at dupuis.ca, lot 376, is an oval of 9.29 carats and has a similarly fine colour, but a different shape.

One of the fascinating features of Tanzanite is its pleochroism. Pleochroism is a directional difference in light absorption, leading to differences in its appearance when examined from different directions. The finest tanzanite is a rich blue when examined face-up, but will show violet colours when examined from different directions.

Tanzanite really shows strong colour differences when viewed from different angles. This is shown to great effect when this same tanzanite is viewed in different positions between polarizing filters in a gemmological polariscope. Above and below you can see the distinct directional differences, face-up and between filters.

Tanzanite in a gemmological polariscope.

Tanzanite in a gemmological polariscope.

Tanzanite in a gemmological polariscope.

Tanzanite in a gemmological polariscope.

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Portrait Cameo

Posted on November 23, 2013 by Duncan Parker
AN ANTIQUE PORTRAIT CAMEO AND GOLD PENDANT/BROOCH, CIRCA 1870

AN ANTIQUE PORTRAIT CAMEO AND GOLD PENDANT/BROOCH, CIRCA 1870

Cameos are small art works often in the form of carved gems. We associate cameos with a white to cream figure on a pink to orange-brown background in the most traditional carved shell cameos. While some of the finest work is in carved shell, historically, agates, chalcedony and other quartz are among favourite media for cameo artists dating back to ancient Egypt.

While these days, cameos are often seen as production line jewels with pointy nosed caricatures of an idealized woman’s head, there are still some exceptional cameo artists working today. Naples is the cameo carving centre of Italy, and some of the workshops have volume carvers working side by side with carvers creating unique one-of-a-kind pieces. It is good to see the art has not been lost.

In the upcoming November 24th Dupuis auction found at www.dupuis.ca there is a most interesting and unique cameo. Lot 201 is a “portrait” cameo. This is a 19th century cameo made as a portrait of an individual, and it is offered with a hand tinted photograph of the gentleman, stated to be George Colyer. Comparing the two images, it is clearly the same man. He has the same curly head, and the same inscrutable look. This is certainly a fascinating antique. For more information, go to: http://bit.ly/17EkKlM

George Colyer Photograph

George Colyer Photograph

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Saint Cecilia

Posted on November 22, 2013 by Duncan Parker
A RUBY, DIAMOND AND GOLD NECKLACE, VAN CLEEF & ARPELS

A RUBY, DIAMOND AND GOLD NECKLACE, VAN CLEEF & ARPELS

November 22 is Saint Cecilia’s day. Saint Cecilia is the patron Saint of Musicians. One of my musical daughters was born on this day too.
Saint Cecilia is usually depicted as a musician, and in this late 19th Century stained glass window in St Paul’s Church in Alliston Ontario, Canada, she carries an instrument. Any artist has the opportunity to interpret their subject, and in this depiction, St Cecilia is wearing what appears to be a lovely ruby neckpiece. If she was depicted today, she might be wearing a ruby and diamond necklace such as lot 37 in the upcoming Dupuis Auction November 24, 2013. The cabochon rubies are bright and lively and are paired with fine diamonds, a beautiful duet, composed by maestro Van Cleef and Arpels. For more information, go to: http://bit.ly/1c9W6cq

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Scallop shells in art, including jewellery art

Posted on November 20, 2013 by Duncan Parker
The delicate pendant, lot 187

The delicate pendant, lot 187

The shell, a symbol of birth, features in art, and, of course, jewellery throughout history. The delicate pendant, lot 187 in the November 24th, 2013 auction features a small cherub, or putto, seated on a scallop shell, and playing a flute.  This image of the shell recurs in some of the great artworks of the world,  including that of Piero Della Francesca, Madonna and Child and Saints, with a gathering in the shadow of a large scallop shell, and Botticelli’s Birth of Venus. The pendant is classic, and interestingly, while many cameos are carved from shell, this one is cut from a banded mineral, agate, and it is referred to as a “hardstone cameo”. For more information, go to: http://bit.ly/1augo1y

Piero Della Francesca

Piero Della Francesca

 

Botticelli's Birth of Venus

Botticelli’s Birth of Venus

 

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Untreated Star Sapphire

Posted on November 16, 2013 by Duncan Parker
Sapphire Ring

Sapphire Ring

Sapphires are amazing for many reasons, including the diversity of colours they may have, the phenomena that may have, and the parts of the world where they may occur.
On average, sapphires are thought to be blue, but they really can be any colour except red (red is reserved for ruby of the same mineral).
This gem has several interesting things to note: It is a star sapphire, with needle-like inclusions oriented in three directions causing reflections to form a 6 ray star, plus it changes colour, purple in incandescent light and bluish in daylight, but it also has bubbles that move around in liquid filled inclusions (one of the bubbles disappears when the gem is warmed to hand temperature, and returns when it cools down). The liquid filled inclusions virtually prove that the gem has not been heat treated, the presence of the star due to inclusions helps to prove it (these would be damaged during the heating process). This sapphire likely originates in Sri Lanka. Lot 244 in the upcoming November 24th 2013 Dupuis Auction. For more information go to: http://bit.ly/HYXndu

Sapphire Ring Inclusions

Sapphire Ring Inclusions

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Unheated Sapphires

Posted on November 16, 2013 by Duncan Parker
Sapphire and Diamond Ring

Sapphire and Diamond Ring

Sapphires and rubies are among the best known gems in the world, the gems flowing through the hands of storybook pirates when they delve into their treasure chests. The gems that are entirely untreated, and are the colour they were when they were mined are very rare, and are most sought after. This pink sapphire has needle-like inclusions, the tell-tale indicators that there has been no heat applied to the gem. These needles, seen through a microscope, in a sapphire are arranged in three directions at 60 degree angles to each other, and are found in this sapphire, lot 80, in the upcoming Dupuis auction, November 24, 2013. for more information go to: http://bit.ly/1bAYQiF

Needle-like inclusions

Needle-like inclusions

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Natural Pearls

Posted on November 16, 2013 by Duncan Parker
AN ANTIQUE NATURAL PEARL AND DIAMOND BROOCH, CIRCA 1900

AN ANTIQUE NATURAL PEARL AND DIAMOND BROOCH, CIRCA 1900

Natural pearls are the original gems. Someone, long ago, was eating an oyster and bit on something, rolling in the palm of our ancient ancestor, it was the first gem, ready to wear, produced through the efforts of a little shellfish. The natural pearl is built up layer by layer, season by season, eventually ending up as this little treasure in lot 72 of the upcoming Dupuis Auction November 24, 2013. For more information, go to: http://bit.ly/17dDULJ

 

Natural pearl

Natural pearl

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Emeralds

Posted on November 16, 2013 by Duncan Parker
Emerald and Diamond Ring at Dupuis Auctioneers

Emerald and Diamond Ring at Dupuis Auctioneers

At auction, you never know what will show up. With the November 24, 2013 Dupuis Jewellery Auction, there is a great range of fine gems. Among the gems, it is interesting to note the majority of the emeralds are from the traditionally most treasured source, Colombia. Their internal features are very identifiable: “Three phase” inclusions consisting of a solid, a liquid, and a gas; a liquid filled inclusion with a small crystal and a bubble. Lot 220 in the auction, for more information, go to: http://bit.ly/19ziRr6

"Three phase" inclusions

“Three phase” inclusions

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Arts and Crafts Pendant

Posted on November 16, 2013 by Duncan Parker
AN ART NOUVEAU AQUAMARINE AND GOLD HEART PENDANT, CIRCA 1910

AN ART NOUVEAU AQUAMARINE AND GOLD HEART PENDANT, CIRCA 1910

Lot 202 in the upcoming Dupuis auction November 24, 2013. This beautiful hand made pendant, dated 1913, seems to be a blend between Art Nouveau and Arts and Crafts. The centre features a panel with a clear cover on the front, and an engraved back with an inscription commemorating a 25th wedding anniversary. The transparent cover allows for the insertion of a photo or other item, perhaps a swatch of colour to pick up a theme in your wardrobe.

I was curious, and tested the clear cover, and it turns out to be rock crystal quartz. As a gemmologist, I was impressed that the cutter oriented it perfectly to be along an optic axis, so it is in a singly refractive direction, and the photo below would not look fuzzy due to doubling. In a polariscope, it perfectly shows interference colours between crossed polarizing filters, and a bullseye optic figure.

Rock crystal quartz in polariscope

Rock crystal quartz in polariscope

Rock crystal quartz in polariscope

Rock crystal quartz in polariscope

For more information about this pendant, go to: http://bit.ly/17BltGs

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