33 Leadenhall Market
London
EC3V 1LR
0207 929 7060
Specific gravity - 3.52
Refractive index - 2.417
Birefringence - n/a - single refraction
Crystal system - cubic
Chemical composition - Pure Carbon (C). Carbon crystallizing in the cubic system, is the hardest known element, by far. Hence, the name 'diamond', from Greek 'adamas' - meaning invincible.
Diamonds can vary alot in price for a given size stone, due to a combination of differences in colour, cut and clarity. When you decide what you want to spend on a diamond you must weigh up what is most important to you and decide on a balance between these factors:
(i) The size - carat weight (or diameter in millimetres).
(ii) The colour - colourless stones range from D - H. From I on the colour becomes more and more tinted to yellowy brown.
As shown in the table, anything I and lower starts to draw colour. Most people wouldn’t notice any colour in I-J stones unless they were placed next to a high colour such as D or E. From K downwards you can see the tint more and more. From H to D the colour improves more and more, D being the highest.
(iii) The make/ cut of the stone - how well the stone is cut and polished.
A well cut stone has its angles cut to take maximum advantage of the internal reflection of the light that enters the stone so as much as possible bounces off the back facets and back out through the table (surface).
(iv) The clarity -
A piqué grade stone has inclusions visible to the eye. How visible depends on the grade - P3, P2, P1 - P1 being the least visible.
An SI grade stone has small inclusions that you would have to look very closely to find. Generally you need a 10x lens to see them - SI3, SI2, SI1 - SI1 being the cleanest.
A VS grade stone has very small inclusions, even under a 10x lens it can be hard to find them. The grade depends on how small the inclusions are - VS2, VS1, VVS2, VVS1 - VVS1 being the smallest, and almost impossible to find with a 10x lens.
The IF grade - internally flawless is reserved for those very special stones that are actually internally flawless.
However, unless the purity is bad enough to interfere with the internally reflecting light and so detracting from the beauty of the stone, it would not affect the overall look as much as the colour and the cut.
Specific gravity - 3.99
Hardness on Moh's scale - 9
Refractive index - 1.76 - 1.77
Birefringence - 0.09
Crystal system - trigonal
Chemical composition - Aluminium Oxide (Al203).
Colourless sapphire is pure aluminium oxide - it is the trace elements that cause the spectrum of sapphire colours we see - such as pink, yellow, violet, green, padparadscha - and red sapphire - which, of course, is ruby. The blue colour is due to traces of Iron (Fe) and Titanium (Ti).
Sapphires come from a group of minerals called 'Corundum' - which is the family name for all colour sapphires, including ruby.
Blue Sapphire Rings.
Padparadscha Sapphire - Good colour, natural padparadscha sapphires are not easy to find. The name 'padparadscha' means 'lotus blossom' in Sinhalese - so called because of the pretty, delicate peachy-pink colour.
Padparadscha Rings.
Specific gravity - 3.99
Hardness on Moh's scale - 9
Refractive index - 1.76 - 1.77
Birefringence - 0.09
Crystal system - trigonal
Chemical composition - Aluminium Oxide (Al203) with Chromium (Cr) traces which cause the red colour.
Ruby, like sapphire, is from the Corundum family, and is, technically, a red sapphire - chromium traces make rubies red, whilst titanium traces make sapphire blue.
Ruby Rings.
Specific gravity - 2.7
Hardness on Moh's scale - 7.5 - 8
Refractive index - 1.57 - 1.58
Birefringence - 0.005 - 0.006
Crystal system - hexagonal
Chemical composition - Beryllium Aluminium Silicate (Be3(Al,Cr)2Si6O18). The green colour of emerald is due to Chromium(Cr), which also causes the red colour of rubies.
Emerald, like Aquamarine, comes from the Beryl family - it is the rich green variety of Beryl.
Emerald Rings.
Specific gravity - 2.7
Hardness on Moh's scale - 7.5 - 8
Refractive index - 1.57 - 1.58
Birefringence - 0.005 - 0.006
Crystal system - hexagonal
Chemical composition - Beryllium Aluminium Silicate (Be3Al2Si6O18).
Aquamarine is from the Beryl family of minerals, like Emerald.
Aquamarines.
Specific gravity - 1.98 - 2.20
Hardness on Moh's scale - 5.5 - 6.5
Refractive index - 1.44 - 1.46
Birefringence - n/a
Crystal system - opal is amorphous - it doesn't crystallize.
Chemical composition - Hydrous non-crystalline silicon dioxide (SiO2 nH2O).
Opals.
Specific gravity - 3.03
Hardness on Moh's scale - 7
Refractive index - 1.62 - 1.64
Birefringence - 0.018
Crystal system - trigonal
Chemical composition - A complex Borosilicate of Aluminium and Alkali, with Iron, Magnesium and other trace elements.
Pink Tourmaline Rings.
Specific gravity - 3.65
Hardness on Moh's scale - 7.25
Refractive index - 1.742 - 1.748
Birefringence - n/a - single refraction
Crystal system - cubic
Chemical composition - Silicate of Calcium and Aluminium (Ca3Al2(SiO4)3).
Tsavorites are a green gem-quality variety of grossular garnet.
Tsavorites, which were first used in jewellery by Tiffany’s in 1974, tend to have their own, unique shade of green - different, but just as lovely as emerald and a pleasant alternative. They are usually clearer and sparkle more so they work well as princess-cuts, whereas emeralds usually wouldn't. Although part of the garnet family, they are rarer and a lot more expensive than the dark red almandine and pyrope garnets you see a lot in Victorian semi-precious jewellery.
Tsavorite Rings.
Specific gravity - 3.35
Hardness on Moh's scale - 6.5
Refractive index - 1.69 - 1.70
Birefringence - 0.0088
Crystal system - orthorhombic
Chemical composition - Silicate of Calcium and Aluminium (Ca2Al3Si3O12OH).
Tanzanite is the blue variety of Zoisite.
The tanzanites - so-called because they were first discovered in Tanzania - vary from a rich, deep violet to more of a pale lilac colour. It is easy to confuse tanzanite with sapphire - tanzanite is mostly more purple, and (blue) sapphires are mostly blue, although some really beautiful tanzanites can look like really beautiful sapphires - not much help! You can also get purple sapphires! Good sapphires would usually be more expensive than good tanzanites but the prices of tanzanites have been rising due to shortage of material and higher demand. Sapphires and tanzanites can easily be identified by their different refractive indices at a good laboratory.
Tanzanite Rings.
Specific gravity - 3.53
Hardness on Moh's scale - 8
Refractive index - 1.63 - 1.64 (pink)
Birefringence - 0.008
Crystal system - orthorhombic
Chemical composition - Aluminium Silicate with Fluorine and Hydroxyl (Al2(F,OH)2SiO4).
Pink Topaz Rings.
Specific gravity - 3.52 - 3.54
Hardness on Moh's scale - 5.5
Refractive index - 1.9 - 2.0
Birefringence - 0.105 - 0.135
Crystal system - monoclinic
Chemical composition - Calcim Titanium Silicate (CaTiSiO5).
Usually more of a collectors gemstone, sphene, also called titanite, is not used much in jewellery because it is relatively soft, at only 5.5 on Moh's scale. It may look like peridot at first glance but is far more interesting and beautiful - in a good gem-quality stone you will see beautiful shimmering golds, greens and yellows merging into each other. You can also see the characteristic doubling of the back facets due to sphene being one of the most highly doubly refractive in the gemstone world, along with zircon. (This means that when light beams hit the stone they are split in two as they are refracted into the stone. Most gemstones have this property - in fact, anything that doesn‘t crystallize in the cubic system, but in most gemstones you can‘t see the effect because it is quite small. In sphene it can be clearly seen as a doubling of the back facets observed through the table of the stone).
A Sphene Pendant.
Specific gravity - 3.7 - 3.72
Hardness on Moh's scale - 8.5
Refractive index - 1.74 - 1.75
Birefringence - 0.0085 - 0.010
Crystal system - orthorhombic
Chemical composition - Beryllium Aluminium Oxide (BeAl2O4)
Chrysoberyl is found in various shades of golden yellow to pale green. It is more commonly seen as cabochon 'cat's-eyes' - the cat's-eye effect, or chatoyancy, being due to rows of tiny parallel crystal needles perpendicular to the bright ray of light.
Chrysoberyl Jewellery.
Alexandrite - is another kind of chrysoberyl. First discovered in the Ural mountains in 1830, it was named Alexandrite because it was found on the day that Czar Alexander II came of age. Alexandrite has the rare ability to change colour from various shades of green in natural daylight to purplish-red or deep red in artificial light - red and green are also the colours of old Imperial Russia.
Alexandrite Jewellery.
