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Learn: Coloured Gemstones

Introduction

Hue

Colour Intensity

Tone

Ruby Colours

Sapphire Colours

Coloured Gemstones

Colour is the single most important factor when evaluating coloured gems.

Generally, the more attractive a gem’s colour, the higher the value. Bright, rich and intense colours are usually more sought after than those that are too dark or too light. However, there are exceptions such as the Padparadscha sapphire which is valued for its delicate pastel hues.

Colour is described in three dimensions: Hue, Tone and Saturation.

THE COLOUR WHEEL

Hues – lie around a circle in the horizontal plane.

Saturation – lies in the horizontal plane, from zero in the center to maximum at the outage edge.

Tone – varies in the vertical plane, from the lightest at the top to the darkest at the bottom.

The more attractive the colour, the higher the value

Hues

Gems with hues that most closely resemble the red, green and blue (RGB) sensors in our eyes are most popular.

The coloured gem trinity is therefore –  Ruby, Emerald and Sapphire.

But a lot of this is also a personal preference and will depend upon an individual‘s taste.

Purple is intermediate between red and violet.

White and black are totally lacking in hue and achromatise (without colour).

Brown is not a hue in itself, but covers a range of hues of low saturation (often in high tone); classic browns fall in the yellow to orange hues.

Diamond halo studs in 18 karat white gold
Diamond halo studs in 18 karat white gold

Lotus Gemology Saturation Scale

Colour Intensity

Intensity is the richness of a colour or the degree to which a colour varies from achromaticity (white and black are the two achromatic colours).

When dealing with gems of the same basic hue position, differences in colour quality are mainly related to differences in saturation, because people tend to be more attracted to highly saturated colours.

Tone

The tone describes a colour’s degree of lightness or darkness, as a function of the amount of light absorbed. White would have 0% tone and black would be 100%. At their maximum saturation, some colours are naturally darker than others. For example, a rich violet is darkest than even the most saturated yellow, whereas the highest saturations of red and green tend to be of a similar tone.

As saturation increases, so does the tone (since more light is being absorbed). However, it can reach a point where increases in tone may result in a decrease in saturation, as colours blacken.

When judging the quality of a coloured gem, the tone is an important consideration.

It is always good to consider the lighting conditions under which it will be worn. Look for stones that look good even under the low lighting you find in the evening or in a restaurant. Also, view gems at arm’s length and look for those that are attractive at distance. Exceptional gems tend to look great under all lighting conditions and viewing distances.

Personal preferences in colour are important, even if specific colours are more popular than others. The colours seen should ideally remain attractive.

 

Ruby

Ruby is a red sapphire. They are both the same mineral, both are corundum, with similar properties. 

Ruby quality and cost are based on its colour. Cut and clarity are factors but in fact, the most desirable and expensive rubies are a little cloudy. Prices increase dramatically as carat weight increases as larger rubies are rarer than larger sapphires and most other very precious gems. 

The brightest and most valuable shade of red is historically called pigeon blood red which is a pinkish vibrant red. In Australia, the more popular colours in the past were fire engine to brick red but at Holloway Diamonds we have noticed a trend to more intense and saturated pinkish-red colours.

Ruby is the birthstone for July and is equal to second hardest (along with sapphire) to scratch after diamond with a designated Mohs hardness of 9. Ruby and sapphire are harder to break than diamond and much tougher than emerald. 

The brightest rubies are those that are very strongly pink-red fluorescent in ultraviolet light. Not all sources of ruby fluoresce but the bee’s knees are those from Burma or Myanmar as it is now known.

Australians tend to spend a fair bit of outdoor time and our cities have more clear sky days than most other big cities. So fluorescence in rubies is a bonus. 

Most rubies (and sapphires) are cut and polished into ovals and cushions because of the barrel-shaped nature of rough corundum crystals. Large rounds and emerald cuts can cost a lot more than ovals. 

Like emeralds, natural rubies are more inclined than most gems to have a lot of inclusions. Rutile needles known as “silk” and other cloud-like inclusions can be a great benefit. Slightly cloudy hazy rubies show up as ‘red on red’ and can appear more beautiful in many types of lighting whereas crystal transparent rubies may appear darker and deeper in poor lighting. Gemmologists use inclusions found in natural rubies to distinguish them from synthetics and imitations. Almost all rubies are heat treated, often in rough form before cutting to reduce the impact of inclusions and make the gems more transparent. Untreated rubies can be identified by the unheated nature of inclusions. They command a large premium overheated rubies and are usually sold with a lab report from a respected gem lab.

The boundary between rubies and pink sapphires is a funny turn of events. Decades ago dealers would offer pink sapphires as rubies because rubies were more valuable. Today top vibrant pink sapphires are more valuable than rubies and lower grade pinkish rubies are often promoted as pink sapphires.

Pigeon’s Blood

A traditional term used to describe the finest colours of ruby: straight Red, glowing colour (not like a red traffic light). Used for rubies coming from Mogok as most of the stones from this mine tend towards purple.

Today, this colour is not unique to Mogok: also found at Mong Hsu (Burma), Vietnam, Mozambique, Tanzania and other localities.

Pigeon’s Blood Ruby

Royal Red

A shade darker than pigeon’s blood. In Burma, this colour was traditionally called “Rabbit’s blood”. These rubies tend to have more iron than the pigeon’s blood type, which cuts the fluorescence and blue transmission giving the stone a darker, pure red colour.

Royal red rubies are typically from Mozambique, Thailand/Cambodia, Kenya and Madagascar.

Royal Red Ruby

Sapphire

Sapphire is a ruby of any colour other than red. Blue is the most common and most well-known, but yellow, pink, purple, orange, yellow and green are also precious and beautiful.

The most desirable blue sapphires are strongly saturated in mid-tones. The most desirable of these have historically been the almost mythical Kashmir sapphire that is slightly hazy intense blue that becomes ‘blue on blue’ because it is slightly opaque and appears blue in many different lightings. 

Like rubies, Australians have moved away from darker blue sapphires to more mid-tones and more saturated blues. Historically Australia was the world’s largest source of deep dark blue sapphires. 

Most sapphires (and rubies) are cut and polished into ovals and cushions because of the barrel shaped nature of rough corundum crystals. Large rounds and emerald cuts can cost a lot more than ovals. 

Sapphire is the birthstone for September. 

Sapphire is one of the two gem-varieties of corundum, the other being ruby (defined as corundum in a shade of red). Although blue is the best-known sapphire color, they occur in other colors, including gray and black, and they can be colorless. A pinkish orange variety of sapphire is called padparadscha.

Australia is still a major source but the most valuable sapphires and collectors’ favourites today are from Burma and Sri Lanka. Other primary sources are Thailand and Laos with more recent sources of Madagascar and East Africa.

Hot Pink

Depending on one’s opinion, this colour can fall into the pink sapphire or ruby category But what makes it “Hot Pink” is the fact that these stones transmit more of the blue to violet wavelengths, due to a relatively low iron content relative to chromium. This results in a bit more bluish red and lots of fluorescence in the red.

Stones displaying this colour typically are from low iron deposits: all the Himalayan deposits (Tajikistan, Afghanistan, Pakistan, Nepal, Burma, Vietnam, and Yunnan) can produce this colour as well as some in East Africa (Mozambique, Tanzania).

Hot Pink Sapphire

Fuchsia

Named after the flower, this is an intense purplish red, redder than hot pink.

Gems of this colour come from a variety of sources including Burma, Sri Lanka, Mozambique, Vietnam, Afghanistan and Tanzania.

Fuchsia Sapphire

Cornflower

Fine blue sapphire are often compared to the colour of the cornflower. The cornflower blues are typically found in the same places as pastel sapphires. In terms of tone and saturation, this colour sits between the lighter pastel blues and the deeper, more intense peacock and royal blues.

Cornflower Sapphire

Peacock

Some of the finest blue sapphires in Sri Lanka have been compared with the colour of the neck or tail feathers of the peacock: an electric blue that can be quite spectacular.

Peacock Sapphire

Velvet

Velvet Blue sapphires are amongst the most highly sought after by connoisseurs. They possess a blue that is almost cobaltian in appearance, these stones are mainly from Kashmir (India), Sri Lanka and Madagascar.

Velvet Sapphire

Royal Blue

The Royal Blue is probably the most difficult to show on screen or in print, as the colour is out of gamut for both printing and most computer monitors – It could almost be compared to a Blue Klein but with depth!

 It is a vivid blue-violet with a deep tone and is epitomized by the fine sapphires from Burma’s Mogok Stone Tract. In addition to Myanmar, we can find Royal Blue sapphires in Madagascar, Tanzania’s Tunduru district and in the smaller sized, sometimes from Pailin (Cambodia) and Nigeria.

Royal Blue Sapphire

Indigo

Traditionally being a dye made from the indigo family of plants, today it is seen as the blue colour of blue jeans. It differs from the pure blues of cornflower, peacock, velvet, and royal in that it is both deep in tone but slightly lower in saturation.

Indigo sapphires are found in many places, particularly deposits derived from basalts such as Thailand, Madagascar, Australia, China, and Nigeria, to name a few.

Indigo Sapphire

Twilight

Resembling the deep blue colour of the sky a few minutes after sunset, twilight blue sapphires come from mainly basaltic sources including: Australia, Thailand, Cambodia, Nigeria, China and Vietnam.

Twilight Sapphire

Padparadscha

A sapphire with a colour said to be a mixture of a lotus flower and sunset, the lovely padparadscha is the most valuable sapphire next to blue.

The original source was Sri Lanka, but fine stones are also found in Madagascar, Tanzania and Vietnam.

Padparadscha Sapphire

Lilac

Named after the flower, these sapphires feature a colour varying from pastel lavender through rich violets.

Found mainly in Sri Lanka, Burma, Tanzania and Madagascar

Lilac Sapphire

Mekong Whisky

This colour of yellow sapphire is in high demand in the Thai market and takes it from the local Mekong Whisky.

These gems come mainly from Chanthaburi, Thailand. Heat-treated gems of similar colour come from Sri Lanka.

Mekong Wiskey Sapphire

Teal

An unusual variety of sapphire is the blue-green teal sapphire, taking its name from the teal duck. These stones generally come from basalt deposits such as Australia, Ethiopia, Madagascar and Thailand.

Teal Sapphire

Pastels

Sapphire does not just occur in high saturations, but also delicate pastel shades. Pastel blues come from Sri Lanka, Burma, Kashmir, Madagascar, Tanzania and Montana (USA)

Pastel Sapphires