of Hatton Garden
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 how much of your hard earned cash you want to spend on your diamonds you must weigh up what is most important to you and decide on a balance between :
(i) The size - carat weight (or diameter in millimetres).
(ii) The colour - do you want the stone to look white or do you mind it looking a bit off-colour/ slightly tinted/ slightly yellowed?
As shown in the table, anything K and lower you will see drawing colour. You probably wouldn’t notice any colour in I-J stones unless you put them next to a high colour such as D or E. From H to D the colour improves more and more, D being the highest.
In the trade a stones colour is determined by placing the (loose) stone in a ‘colour card’ to isolate it from outside influences and judging its colour from experience and/or placing a known colour diamond beside it for comparison. The laboratories also use comparison stones like this : comparing the stone to a known colour is a critical part of accurate grading.
(iii) The make/ cut of the stone - on examining the stone closer you would notice the make or cut. If it were a thickly cut, lumpy stone or far too shallow you would notice this as it wouldn’t sparkle like a well cut stone. As a diamonds main attraction is its sparkle the make becomes very important.
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). A badly cut stone should generally be cheaper than a well cut stone, all other things being equal. It may be difficult to know if you want to compromise on this factor without comparing different makes in the same size stone at the same time, which may not be practical. It’s easier to let your jeweller be the judge of this as they would have had alot more experience.
(iv) The clarity -
Would you mind being able to see flaws (piques/inclusions) in the stone? (A pique stone - P3, P2, P1). Or would you rather not be able to see anything in it on close examination (SI3, SI2, SI1). Or, maybe, you prefer to know that it is very clean, even under a 10x lens (VS2, VS1, VVS2, VVS1). Or even internally flawless (IF). The price obviously gets higher the cleaner the stone is.
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.
Sometimes people put the clarity of the stone as the first and most important consideration. To me, personally, I would always put the colour first - the most important thing for a diamond is to look beautiful ie. to be as white and as sparkly as possible (or fancy colour and sparkly), therefore it naturally follows that the colour and the cut should be the most important considerations. The colour is the first thing you see when you look at a diamond - if it is a cape (off-colour) stone as opposed to a white diamond you notice this straight away. Although, all three factors - colour, cut and clarity - affect the beauty of a diamond and the price you pay for it.
However, if this sounds like alot to think about when you go out to buy a diamond, just have some faith that your independant jeweller will be one step ahead of you and will already be taking into consideration the colour, cut and clarity for the size and/or price range you have given them.
We keep all qualities of diamond because there is a market for all qualities at the right price, but we generally prefer to buy eye-clean/SI, high colours with a good make. We keep some VS’s and some certificated.
The round brilliant cuts we’ve used in the jewellery vary a lot in purity, but are mostly high colours. We always use high coloured, eye-clean diamonds to go with the coloured stones. The stud earrings and single-stone rings generally have low purity stones to keep the prices down. If you wanted a better quality stone this is no problem but it will be more expensive.
Nearly all of the fancy cuts (radient, princess, emerald, asher, pear, marquise & oval) are high colours - mostly D, E & F, one or two are lower.
The fancy colours are all natural colours. Most will have a cert. stating their natural origin, except for a few small stones under 0.50ct.
No treated or synthetic diamonds are used.
All the coloured stones used are sourced equally for their colour, cut and purity.
We aim for the perfect colour for each gemstone: if a stone is too dark it can look dull - if it doesn’t have a vibrant colour or much sparkle (although some sapphires can be very dark and still look beautiful). If it is too pale it can look wishy-washy (but then pink topaz is a very pale pink and this delicate colour together with its gleaming lustre are what makes it so pretty).
A fine gemstone should generally have some depth - you should be able to see into the stone. Some stones can be cut too flat so you see right through them and you can only see colour around the edges - they may look nice and clean but this is not an attractive cut for most fine gemstones. A lot of mid-blue and lighter Ceylon sapphires are cut particularly deep to capture that beautiful, sparkly cornflower-blue colour.
It is acceptable for gemstones to contain inclusions/ piqué as long as it doesn’t interfere with the beauty of the stone. Too much pique can also make a stone look dull and cloudy even if it’s a good colour, but then again, sometimes, the inclusions can enhance the stone.
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 known as 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.
The blue sapphires we use are either dark, rich velvety-blue material or sparkly Ceylon cornflower mid-blue material which are usually cut deeper to get that amazing colour. The pink and yellow sapphires are of the same sparkly Ceylon material.
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. You may see darker orange, opaque material being sold as padparadscha - it most definitely isn't.
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.
The rubies we use are the purest, richest reds we can find, ie. no pinky reds or cloudy reds, and no brown or purple tinges - just deep, rich, blood red as good rubies should be.
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.
All the emeralds we have are a beautiful rich green colour - some more of a yellowy-green, others more bluey-green. Inclusions in emerald, known as an ‘emerald garden’, often enhance the stone, as long as there aren't so many as to make it look opaque.
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.
With aquamarine, we aim to buy that beautiful silvery-blue material. If it's too dark it can look cheap and blue-topazy - too pale and it is cheap, and everywhere. Aquamarine material is mostly found very clean, which makes it all the more important that it is well cut. Gem quality stones look particularly beautiful when they have inclusions we call ‘rain’ - these are tiny parallel tubes running through the stone that reflect like rain in sunlight.
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 have a crystal system.
Chemical composition - Hydrous non-crystalline silicon dioxide (SiO2 nH2O).
There are several different theories of how the opal transmits those beautiful flashes of colour.
My favourite is that it is caused by the fact that - (this is much easier to visualise than write about!) - as opal is made up of tiny amorphous silica spheres, and the spheres are all different sizes - different sized wavelengths of light are reflected back - the tiny wavelength of blue light escapes through the tiniest spheres so we don't see it again, we see the larger rays of oranges and reds which are reflected back at us, because they can't fit through - and larger spheres absorb the larger lightrays at the other end of the spectrum, so we see more blue light. So at different angles we see different colours as the shape and size of the spheres, and the gaps between them, vary from a single view point as the stone is turned.
Anyway, it's to do with the variation in size of the tiny spheres, and the different size wavelengths of all the pretty colours that make up a light ray.
The opals we use are fine transluscent material with lots of colour-play. You can substitute very cheap opals: as low as about £30/carat for a one-carat stone but they would be very bland - milky with hardly any colour-play. The finest black opals, with lots of electric colours can cost thousands of pounds per carat - but they look stunning!
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.
The pink tourmalines, also known as rubellites, were selected for their bright cerise-pink colours. They are beautiful, and being only semi-precious, are not as expensive as some coloured stones. Some of them have a lovely chequered cut, which enhances the life of the stone. Tourmalines come in many other colours besides pink - dark green being the commonest and cheapest. The trigonal crystals, you may have seen in crystal and gem shops, with one end green and the other end pink - are tourmaline.
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.
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 at a good laboratory here in Hatton Garden.
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).
When we buy pink topaz we look for that pretty, delicate baby-pink colour with no tinges of brown. Pink topaz is mostly very transparent and clean with a glossy lustre unlike most other gemstones. The pink sapphires sparkle - pink topaz glistens.
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).
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.
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 the colours of old Imperial Russia.
