Sapphires are most often associated with the colour blue but they can be of practically any colour … except red. From colourless to black or white and almost anything in between. Sapphire may even be mottled thereby giving it a ‘marbled’ look. Sapphire may also produce a star-like effect called asterism. When this star effect is clearly visible and the star well centred, stones may command a rather high price. Pinkish or purplish star sapphires normally command the highest prices while sapphires of other colours commanding lower ones. Some colours are only achievable thanks to a colour treatment by ‘doping’ the stone with an impurity (such as led) or by subjecting it to irradiation. Manufacturers may start with a colourless stone to impart it some colour, or they may start from an off-colour stone, say a stone that is too dark, and irradiate it to produce a near-black coloured stone. Blue, however, remains the most sought-after colour and the most expensive when the clarity, cut, and colour are just right.
Sapphires are said to have very good wearability as stones of the corundum family are the second hardest gemstones after diamonds.
Appearance to the naked eye
Few buyers go into a retail stores with a loupe and gemological tools in tow. They rely on how appealing the gem is to them purely from a visual perspective. And this is also the case when you are out to dinner or in a social gathering; it is highly unlikely that the other guests are using sophisticated laboratory tools to admire your ring; instead they are relying on what they see with their naked eyes. That is why we want to spend a bit of time explaining what it is you can expect to see just by eyesight.
As mentioned above already, sapphires may be of practically any colour except red. While blue remains the most sought-after colour.
The finer quality stones will be transparent, but buyers will also find specimens that are translucent, thereby giving them a jade-like appearance, or opaque. Stones that are translucent or opaque are rarely used in rings, but can be used in pendants, cuff-links, or carved into an amulet or brooch.
As mentioned above already, sapphires may be mottled with white streaks, which gives them a marbled look. This is especially true if the stone is of a greyish colour. They may also present a star-like formation. But another effect is something called colour-change, which means stones may show one colour under one type of lighting (say indoor incandescent lighting) and another colour under different lighting conditions such as outdoor sunlight. This is due to a technical feature of sapphires called pleochroism. Sapphires present a rather strong pleochroic effect.
Sapphires are actually quite well known for their inclusions so, with commercial-quality product, the presence of inclusions needn’t necessarily affect prices negatively too much unless the inclusions are of such quantity or severity that they especially detract from the stone’s visual appearance. Most often, inclusions natural to sapphires are needle-like spires of rutile. It is these rutile inclusions that are the reasons for the star or cat’s eye effect some stones may present.
- Sapphires are the second hardest gemstones after diamonds.
- Colourless sapphires are sometimes used as a diamond simulant.
- Sapphires have many industrial uses; including their use in the manufacture of wristwatch face crystals. Given their durability, they are used in the manufacture of some scientific equipment, infrared optical components, high-durability windows, wristwatch movement bearings and as insulating elements in solid state electronics to name just a few.
- The finest quality sapphires are blue with a slight purplish tint (15% colour saturation).
- Sapphires may present a six-point or twelve-point star formations.
- Refractive Index:
lower value ranging from 1.760 to 1.766
upper value ranging from 1.768 to 1.788
0.008 to 0.009 range
- Optic Character:
double refractive, uniaxial, negative: U-
- Spectral Absorption:
extinction band in the blue end of the spectrum in the 450nm range.
- Hardness (Mohs scale):
- Specific gravity:
3.95 to 4.03 range