OPTICAL EFFECTS IN COLORED GEMSTONES

March 01, 2015

Some times colored gemstones present some very striking optical effects.

ASTERISM is a crossing of lines or bands shaped as a star, and we can find the shape with four and six arms. It is typical of sapphires or rubies, although it can also appear in other gems (diopside – 4 arms). These are caused by really small inclusions that are orientated following the gem's crystal structure, and they form those nice stars or asterisms on the cabochon gems.

ADULARESCENCE is an optical phenomenon that is produced on moonstones. It produces a bluish or milky-white colour due to the lamellar structure. The moonstones present some very beautiful inclusions that look like centipedes.

Imagine a stone with a lot of inclusions, really small layers of mica or hematites - the effect that the reflection of light produces on it is what is called AVENTURESCENCE : when you move the stone some beautiful reflections can be observed. The photograph I have on Facebook showing one thousand colours corresponds to a felspar (Sunstone).

ASTERISM	ADULARESCENCE	AVENTURESCENCE

When the inclusions are like small needles orientated in parallel, and the gem is cut in cabochon, we can notice the CHATOYANCY – CAT'S EYE EFFECT. A very similar effect can be seen in the eyes of felines. The alexandrite can present a beautiful chatotancy, which can also be seen in tourmalines, aquamarines, etc.

There are occasions where the gems are made up of very fine layers or where they feature many fissures or delaminations, which means that when light is reflected on them there are interferences and we see something like a “rainbow”. This effect is called IRIDISCENCE of which the ammolite is a beautiful example.

The effect that looks like a bluish-greenish metallic reflection in the labradorites (felspar) is known as LABRADORESCENCE. It is also due to the interaction of light with the thin layers of felspar.

CAT'S EYE EFFECT	IRIDISCENCE	LABRADORESCENCE

Who hasn't heard of opals? PLAY OF COLOURS: The play of colours appears in noble-opals due to their structure- if you get an opal and move it around, you will see a really beautiful play of colours. The more colours of the spectrum they show the more valuable opals are considered to be. There are certain colours that are valued more (as they are more infrequent), also if the opal stone is dark (black opal – that's why they are dyed), as well as when the part of the gem where the coloured reflections appear occupies the highest proportion of the gem. Opals are a whole world of their own!

And then we should talk about the CHANGE OF COLOUR. The alexandrite is said to be an “emerald” during the day and a “ruby” at night- depending on the type of light that falls on it, we will see one colour or the other. Sapphires can also present changes in colour, as can garnets, etc. It is a strange phenomenon, so the more dramatic the change of colour is and the faster it happens on the gem, the more valuable it will be.