April 25, 2020
Some times colored gemstones present some very striking optical effects. This is due to characteristic structures, defects or even inclusions, gems can show different effects such as:Contents
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.
Adularescence in a moonstone.
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.
Gems that can show aventurescence: Sunstone, Aventurine, Golden Obsidian.
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. Chrysoberyl can present a beautiful chatotancy, which can also be seen in Tourmalines, Aquamarines, Apatite, Beryl, Diopside, Quartz, 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.
Labradorescence in labradorite
Who hasn't heard of opals? PLAY OF COLORS: The play of colors 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 colors. The more colors of the spectrum they show the more valuable opals are considered to be.
Australian Solid Matrix Boulder Opal 3.06 ct. This beautiful stone is like a coffee bean with vibrant colored veins between it. This opal display a magnificent play of color (red, orange, yellow, green, violet).
There are certain colors 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 colored 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, Change Color Diaspore, 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.
Optical effects in colored gemstones are beautiful!.
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