Knowledge Centre > Gemmology > Gemstones > Gemstone Phenomena

Phenomenal Phenomena - Opting for Optical

Art Deco black opal and diamond ring, circa 1920 hatton garden
Art Deco black opal and diamond ring, circa 1920.
Ref: 27852

Thursday 20th July 2023

Phenomenal Phenomena - Opting for Optical

By Luke Cairns

“All that we see or seem is but a dream within a dream.” – Edgar Allan Poe

In this job it is very easy to become accustomed to the extraordinary. Every day we are fortunate enough to handle some of the finest jewels mankind has ever made, incredible pieces that punctuate the most important moments of people’s lives; whether it be a gift given at the birth of a newborn, rings exchanged to signify a shared love, or a treasure chosen to commemorate a loved one, sorely missed.

Less frequently however, do we come across a phenomenon within the already phenomenal. I am of course referring to gemstone phenomena, as it is known, which encompasses a variety of optical effects that can be found in some of our most beloved gemstones. Mother nature’s mysterious charm dictates which gemstones will be touched by these magical properties. An array of visual characteristics that enhance the already beautiful gemstones, imbuing them with an added layer of wonder. These are asterism, chatoyancy, adularescence, play-of-colour, iridescence and colour-change.

“Oh, don’t let’s ask for the moon. We’ve already got the stars!” – Bette Davis

Asterism, or as it is more commonly known, the ‘star’ phenomenon, is one of the most sought-after optical effects that occur naturally in gemstones, with the most prized being those occurring in sapphires and rubies. Nevertheless, stars can also be found naturally in spinel, diopside, beryl and quartz. This captivating quality is caused by tiny needle-like inclusions known as ‘silk’ that, once exposed to bright light, will reflect the light in a burst of rays across the stone. When these stones are cut ‘en cabochon’, meaning a smooth, domed polish with no facets, the light can form four, six or very rarely up to twelve rays, all spreading from the centre of the stone. A famous example is the Star of India, a 563.35 carat milky-blue star sapphire that despite its name originated from Ceylon (modern day Sri Lanka), affectionately known as the ‘jewel box of the Indian Ocean’.

“Cats are connoisseurs of comfort.” – James Herriot

If astronomy is not your cup of tea, would a fascinating feline phenomenon take your fancy? Known in the industry as chatoyancy, the French word for ‘shining like a cat’s eye’, or ‘cat’s eye’ for short, it is an extraordinary optical effect that occurs when some gemstones, cut en cabochon, reflect a narrow line perpendicular to the observer’s line of sight. Much like asterism, this is caused by tubular or needle-like inclusions that are tightly packed in one direction throughout the stone. If the term ‘cat’s eye’ is used on its own, it will always be in reference to chrysoberyl, the most common gemstone to harbour this property. It does however occur in other gemstones such as tourmaline, beryl, demantoid garnet and quartz.

Adularescence is a phenomenon also caused by inclusions, however unlike our previous gemstones, no rays are formed. In this case the inclusions reflect light throughout the stone, creating a hazy glow just below the surface.  Perhaps the most popular adularescent gem is the moonstone, celebrated for its bluish-white milky radiance.

Play-of-colour is a phenomenon that is only found in opals, which is, according to the Gemological Institute of America (GIA), the world’s most popular phenomenal gem. Play-of-colour refers to the flashes of colour exhibited by opals, which break up light into distinct spectral colours, caused by the grid-like structure of the stone.  This can occur in many of the different varieties of opals—black, white, and fire opal, each named for the base colour of the stone.  Some of the rarest and therefore more valuable opals are those which display all the colours of the rainbow, most notably red, against a black base colour.

Iridescence is the term for an object’s ability to change colour as it moves.  Mother-of-pearl is one such substance which exhibits iridescence, as do iris and fire agates. Like opals, pearls also have their own phenomenon, a variant of iridescence, which is called orient. These organic gems typically have a base colour—most typically white or cream—as well as an over tone—such as pink, blue, purple, or green.  Orient is the name given to a pearl with an iridescent or multicoloured overtone.

One of the most amazing gemmological phenomena, colour-change refers to gems which change colour in different types of light.  The most famous of these is alexandrite chrysoberyl, which changes from reddish purple in incandescent light to bluish green in fluorescent light. Another gemstone with the ability to change colour are sapphires, which we are privileged to have within in our collection, including some incredible Ceylon varieties.

Exceptionally rare, some stones exhibit multiple phenomena. A number of star and cat’s eye gems—such as sapphire or alexandrite—can at the same time be colour-change. Whatever the effect or combinations thereof, phenomenal gemstones lend the jewels in which they are set an allure beyond the brilliance they already possess.

If you choose to opt for optical, if you have a preference for the phenomenal, if you want to experience the extraordinary, come and see us… You won’t be disappointed.


For our existing customers, did you know that you can log in to your Account on the website?

In this section you can book your annual clean and check – by appointment only, edit your contact details, request an updated valuation and find out more about our exclusive wedding band service.

Our extensive collection of ancient, early, antique and vintage jewellery can be viewed online or in store. Found the piece of your dreams? Don’t let it get away – secure it with a 20% deposit, the balance payable within 6 months.

Mauve star sapphire diamond cluster ringberganza hatton garden
Mauve star sapphire and diamond cluster ring, circa 1920.
Ref: 18070
Oscar Heyman Brothers chrysoberyl diamond ring berganza hatton garden
Oscar Heyman Brothers chrysoberyl cat's eye and diamond ring, American, circa 2019.
Ref: 22118
Art Deco black opal and diamond ring, circa 1920 hatton garden
Art Deco black opal and diamond ring, circa 1920.
Ref: 27852
Moonstone diamond and emerald cluster ring, circa 1890 hatton garden
Moonstone diamond and emerald cluster ring, circa 1890.
Ref: 28216
Edwardian pink star sapphire cluster ring berganza hatton garden
Pink Ceylon star sapphire and diamond coronet cluster ring, English, circa 1905.
Ref: 25047
Oscar Heyman chrysoberyl and diamond ring berganza hatton garden
Oscar Heyman Brothers cat's eye chrysoberyl and diamond ring, American, circa 1954.
Ref: 16963
Fred diamond and mother of pearl cluster ring, hatton garden
Fred diamond and mother of pearl cluster ring, French, circa 1970.
Ref: 26074
front view Oscar Heyman Brothers star sapphire ring, circa 1942.
Oscar Heyman Brothers star sapphire cocktail ring, American, circa 1942.
Ref: 20770
Star sapphire solitaire ring, Polish, circa 1940. Hatton Garden
Star sapphire solitaire ring, Polish, circa 1940.
Ref: 28147
Black opal solitaire ring, circa 1930. Hatton Garden
Black opal solitaire ring, circa 1930.
Ref: 27272
Berganza on Facebook Berganza on Instagram

Email: | Opening hours: Monday-Friday 10am-5pm, Saturday 11am-5pm | All Stripe currencies supported

Connect with us

Berganza on Facebook Berganza on Instagram Berganza on Pinterest Berganza on youtube Berganza on linkedin

Signs up for regular emails on our new acquisitions, news and features:
Updated 19/05/2024 at 1:34PM

© Berganza Ltd 2024