Only diamonds, rubies, emeralds and sapphires make the cut as precious stones - all other gemstones are considered semi-precious.
This distinction, created in the mid-19th century, is unscientific but reflects the rarity as well as hardness of the stones.
Demand for precious stones has jumped 25 per cent since 2008 despite the recession, say industry experts.
Patrick Normand, managing director of high-end Swiss jeweller Chopard (Asia), says: 'Exceptional coloured gems have actually benefited from the recent global financial crisis as they represent a safe asset and investment.'
While white diamonds have always been the most popular, he notes that in the last three years, 'coloured stones have got many people talking because of their rarity and intense colours'.
Rubies and sapphires have become especially popular in the past year.
Tanja Sadow, dean of the Jewellery Design & Management International School here, says Asians appreciate the blues of sapphires and reds of rubies and notes that 'rubies always see a spike in sales around Valentine's Day due to their colour'.
All coloured gemstones are graded using four basic parameters which are known as the four Cs - colour, cut, clarity and carat.
Colour is the most important for all except diamonds, for which cut is the most prized. The more intense the colour, the more valuable the stone.
EMERALDS
A top-grade emerald is a rich and bright green - not light or dark. Neither should it contain tinges of yellow or blue.
As emeralds inherently have numerous inclusions and fissures, clarity is judged by the eye.
So long as it has no visible inclusions, it is considered flawless.
Oiling emeralds with cedar oil to reduce the appearance of blemishes is an industry practice and does not diminish their value.
However, any other oiling treatment, such as using synthetic oils, can slash an emerald's value by as much as half.
RUBIES
Named after the Latin word for red (ruber), rubies vary from pink to blood-red. The brightest and most valuable are 'pigeon blood' rubies from Myanmar, so-called because they appear to be the colour of the blood of a freshly killed pigeon.
The stone's colour is caused by the element chromium.
Pink, orange and purple are the common secondary hues in rubies. Of the three, purple is most preferred because it reinforces the red, making it appear richer.
Setting a purplish-red ruby in yellow gold neutralises the purple, so the stone appears to be pure red.
After colour comes clarity. As with diamonds, a clear stone commands a premium.
Like emeralds, rubies are commonly treated to improve their colour, texture and translucence. The most acceptable treatment is applying heat to enhance redness through the removal of purple tinges and blue patches.
SAPPHIRES
Although blue is the most well-known hue, sapphires come in a variety of colours including yellow, green, grey, black and pinkish-orange.
Blue sapphires are the most prized and are evaluated based on the purity of their primary hue. Intense cornflower blue is the ultimate blueblood.
Blue sapphires, favoured for their classic glamour, are said to be the hottest precious stone in the market now.
Purple, violet and green are the usual secondary hues found in blue sapphires.
Violet and purple can contribute to the overall beauty of the colour, while green is considered a distinct negative as it dilutes rich blue tones.
Blue sapphires with no more than 15per cent violet or purple are said to be of fine quality, but those with any fleck of green are deemed inferior.
TAKING A SHINE TO OTHER GEMS
They may lack the lustre of diamonds but semi-precious stones are having their turn in the sun.
Topaz, rose quartz, opal and amethyst are four of the most trendy gemstones now.
Tanja Sadow, dean of the Jewellery Design & Management International School, says more than 3,800 mineral types have been discovered and about 80 new minerals are discovered each year.
More than 100 of these are currently used in jewellery.
'For semi-precious stones, it is the overall design that dictates the price rather than a specific gem,' she says.
Jewellers and customers are going crazy for their colours.
Home-grown chain TianPo Jewellery has seen a 25 per cent spike in demand for coloured gemstones since 2008, says its marketing manager, Joanne Lim.
A growing trend is to integrate semi-precious gems with more exquisite stones, such as pairing quartz with white gold and sapphires, she notes.
Says Francis Foo, founder of Loang & Noi: 'More customers are looking for unusual stones to stand out from the crowd and the unique designs of these pieces allow them to showcase their individuality.'
The luxe home-grown jeweller uses unconventional semi-precious stones such as tsavorite, moonstone and peridot in its designs.
ROSE QUARTZ
Quartz has a milky pale pink to rose-red hue. The colour is due to the presence of the trace elements titanium, iron or manganese.
It used to be unpopular because it was perceived to be too clouded by impurities but experts say younger buyers are embracing rose quartz due to its affordability and the trend for pink and purple gemstones.
TOPAZ
Pure topaz, a silicon-based mineral, is colourless and transparent but is usually tinted by impurities.
Typically, topaz is wine-coloured, yellow, pale grey, reddish-orange or bluish-brown.
It can also be white, pale green, blue, gold, reddish- yellow or translucent. Its golden brown to yellow forms are commonly used in jewellery and are most prized because they are the rarest.
Blue topaz, another popular colour of the stone, is rarely natural and is produced by irradiating and then heating clear crystals.
OPAL
Opals have a similar silicon composition to quartz but have a more variable water content, which result in differences in colour and the degree of milkiness.
Pure opal is colourless but like topaz, its colours can vary widely.
These include white, grey, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown and black, due to the presence of impurities.
Black opal is the most rare and valuable and white and green the most common and affordable.
AMETHYST
Amethyst is a violet variety of quartz. Its name comes from the ancient Greek word 'amethustos', meaning 'not intoxicated'.
In ancient times, the stone was believed to be able to protect its owner from getting drunk.
The colour range of amethysts is wide, from pale lilac to a rich, deep violet.
When shopping for the purple stone, ask for the deep purple hue called 'Deep Siberian', considered the most ideal in amethyst jewellery.
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This article was first published in Urban, The Straits Times.
