Gemstone Guide
Birthstones
January - Garnet
February - Amethyst
March - Aquamarine
April - Diamond
May - Emerald
June - Pearl
July - Ruby
August - Peridot
September - Sapphire
October - Opal
November - Citrine
December - Turquoise
Brief guide on precious stones
Diamond
Hardness: 10
Diamond is the hardest natural substance and the most valued gemstone. They are the primary gem measured in carats. More than half their weight is lost during the cutting pricess. The value of a diamond varies according it's colour and transparency. Perfectly colourless and transparent gems are the most expensive and rare, since most diamonds have impurities.
Fancy colours are also greatly valued such as natural red, blue and green. Diamonds can be coloured artificially using a process called Irradiation.
Sapphire
Hardness: 9
The usual range of sapphire colours is from very pale blue to deep indigo. Other colours are pink, yellow, white, green, orange, black, grey and purple.
The majority of sapphires today are heat treated to improve their colour and clarity. Naturally coloured stones are more expensive.
Ruby
Hardness: 9
Rubies exist in many shades of red, from pinkish to brownish red. Like sapphires they may have needle like inclusions that appear as a slick sheen.
As rubies are very rare the very large stones have earned higher prices per carat at auctions than flawless diamonds.
Emerald
Hardness: 7-8
Practically all emeralds possess inclusions. A transparent stone with a pure green colour is the ideal. Emeralds are treated with oils and resins during cutting and polishing to fill fissures and produce a better finish.
Emeralds must not be immersed detergents or cleaned in ultrasonics and should be treated with great care.
Aquamarine
Hardness: 7.5-8
A top quality Aqua should be clear and free from inclusions. The most popular are clear blue avoiding a tinge of yellow, green or grey. As the aquas colour us usually pale only the larger stones display any depth of pigment.
Tourmaline
Hardness: 7-7.5
Tourmaline displays a greater range of colours than any other stone. These include pink/red, blue, colourless, brown, yellow-green, violet and black . Often a crystal will have different colours at its two ends or one in the centre and another outside.
Opal
Hardness: 5.5-6.5
This is made from hardened gel of silica & contains up to 10% of water. It is a very delicate stone. If it comes into heavy contact or is exposed to heat it can crack.
The more colour 'Fire' in the stone the more value it has.
Tanzanite
Hardness: 6
Tanzanite is the blue/purple variety of the mineral zoisite. was discovered in the Mererani Hills of Northern Tanzania in 1967
In its natural state is usually a reddish brown color. It requires artificial heat treatment to 600 °C to bring out the blue violet of the stone
Garnet
Hardness:6.5-7.5
The most common colour is red, brownish-red. The most expensive varieties tend to be green or yellow.
The green garnet also known as Tsavorite is mainly found in East Africa. It is similar in colour to the Emerald but its higher refraction and dispersion give its a much higher sparkle.
Pearl
Pearls are formed when a foreign substance slips into the oyster between the mantle and the shell. This irritate's the mantle. The oyster's natural reaction is to cover up that irritant to protect itself. The mantle covers the irritant with layers of the same substance that is used to create the shell. This eventually forms a pearl.
Amethyst
Hardness: 7
The colour of anethyst is unstable and can diminish with prolonged exposure to sunlight. Amethyst can also be heat treated to produce the yellow of the quartz family Citrine. The deeper purples in Amethyst are the most valuable.
Sometimes it is part of a mixed crystal. When it bands with Citrine it is called Ametrine.
Citrine
Hardness: 7
Natural bright yellow citrine is the rareish of the quartz family. Today most Citrine is artificially created from heat treated Amethyst. As it is heat sensitive it should be protected from excessive heat or light.
Topaz
Hardness: 8
Pure topaz is colorless and transparent and is usually tinted by impurities. Typical topaz is wine, yellow, pale gray, reddish-orange, or blue brown. It can also be made white, pale green, blue, gold, pink (rare), reddish-yellow or opaque to transparent/translucent.
Peridot
Hardness: 6.5
Peridot only occurs in only one color, an olive green. The intensity and tint of the green, depends on how much iron is contained in the crystal structure. The color of individual peridot gems can vary from yellow- to olive- to brownish-green. The most valued color is a dark olive-green.
Turquoise
Hardness: 5-6
This stone is sensitive to sunlight, soap, water, dry air,perspiration and grease.
Iolite
Hardness: 7-7.5
The name "iolite" comes from the Greek word for violet. Another old name is dichroite, a Greek word meaning "two-colored rock",
Durable if slightly brittle.
Blue John
Hardness: 4
The only source of Blue John is in Castletion in Derbyshire. The larger veins in the the Derbyshire mines have been overworked and no longer exist, so decorative objects made today are smaller. It is a fragile stone so care must be taken.
Spinel
Hardness: 8
Although the red spinels are the most renowned it also occurs in pink, blue, green and even black.
