Fashion is easy to understand, it's all about colour popularity; if no one finds a gem's colour attractive, then its rarity really doesn't matter. In general, intense colours are more favoured than those that are dark or light, but there are exceptions, for example, that Victorian favourite, Rose de France Amethyst.
Self Coloured Idiochromatic : A colouring element is incorporated into the mineral's crystal structure, always giving it a characteristic colour. For example, Peridot will always be shades of green from its high iron content, which comprises percent of its total mass. In their pure state, 'other coloured' minerals are colourless, for example Sapphire.
Most things get their colour due to how they react with light. Some gems, such as Opal, get their colours physically as light is refracted in flashes of colours that change with the angle of observation 'play of colour'. But for most gems, and even most objects, our perception of colour involves an amazing transformation. As revealed by rainbows, white light is comprised of the individual colours of the spectrum: blue, green, orange, red, violet and yellow.
When white light passes through a gem, some of these spectral colours are absorbed. The spectral colours of white light least absorbed combine to produce the colour of the gem.
This absorption of certain colours is called the 'selective absorption of light' and is always the same for an individual gemstone. Interestingly, many minerals are actually colourless in their pure form, and it is the inclusion of impurities in their structure which leads to their colouration. Generally speaking, we observe an object as coloured when it absorbs some wavelengths of visible light, but not others.
Different colours of light have different wavelengths, so the exact wavelengths that are absorbed will affect the colour that the object appears. For example, an object that absorbs all wavelengths of visible light that pass through it, but does not absorb red light, will appear red. Why does this absorption of light occur in the first place? This is dependent on the elements present in the structure of the gemstone.
Conversely, the transition metals the large group of metals in the centre of the Periodic Table are capable of absorbing coloured light. Transition metals have this capability because they have electrons in d orbitals. Orbitals are essentially regions of space around an atom in which electrons can be found; they can have different shapes and energy levels.
The d orbitals in transition elements are partially filled, and this mean the unpaired electrons therein are capable of absorbing visible light in order to promote the electrons to a higher energy level.
When they do this, the wavelength of light they absorb is removed from the light completely. Rubies are also found in different shades of red, while the color known, as "pigeon's blood" is the darkest and richest red available, which is very powerful. This deep red ruby is mined only in Myanmar. When precious stones are found in mines, they do not look as attractive as they should be. Stones are usually polished, carved and cut into different shapes and sizes for it to acquire its beauty.
According to the polish of the stone, the value increases. It is a common notion that the deeper and richer the color of the stone, the more precious and valuable it is. For instance, bright white pearls, green garnets, royal purple amethysts, yellow topaz, deep red corals are the most precious and valuable gem stones existing. A lighter colored gemstone, like a light green garnet isn't as precious as a parrot green garnet.
It is very difficult to obtain the richest color of a stone, as they are very rare. Thus most of the people settle with lighter colors, as they are easily available and very affordable. These days, people prefer a lighter garnet, or a lighter purple amethyst, as it is next to impossible to afford or find the "perfect" green or purple. As long as the gemstone is powerful and attractive people are content. Sapphires are also a form of corundum.
Sapphires can be a variety of different colors, such as yellow, purple, green, orange, and even multicolored or colorless. However, most commonly, sapphires are blue. Just like rubies, sapphires have a hardness of 9 on the Mohs scale. Apart from their ornamental purpose, they also serve in various technological devices, most commonly in timepieces.
Unlike the previous two gemstones, emeralds are a variety of the mineral beryl. Its green color comes from chromium. Alternatively, it can come from vanadium.
However, European traders do not include the vanadium variant into the traditional emeralds. Emeralds are less hard than diamonds, rubies, or sapphires, scoring only 7. There are synthetic emeralds as well.
The hardest naturally-occurring mineral, diamond has the hardness of 10 on the Mohs scale. It is pure carbon crystal, and in its purest form, it is transparent and colorless. If there are some impurities in the stone, it can take on various hues and colors.
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