Play of Color
by Nasrina Parvin Khan
A well-known Play-of-color gemstone is opal. Opal is an amorphous from of silica (sio2, nh2o). It is made up of sub microscopic spheres of silica with a little amount of water and air trapped in the spaces between these round spheres.
What is Play-of-color?
Play-of-color is a unique special optical phenomenon. This phenomenon is usually exhibits by the gem opal. The iridescent color effect in opal described as Play-of-color. Color of rainbow can be seen in gems by this optical phenomenon. But the displayed colors are unrelated to the gem’s own body color. So, we can guess that the structural pattern of gem is responsible for this colors creating optical phenomenon. Due to the structural pattern the light interfere or diffract in gems and produce the color phenomenon named as Play-of-color.
Play of color in Opal
A well-known Play-of-color gemstone is opal. Opal is an amorphous from of silica (sio2, nh2o). It is made up of sub microscopic spheres of silica with a little amount of water and air trapped in the spaces between these round spheres. The amount of water in opal may be 3% to 21%. But the precious opals have 6% to 10% water content usually.
Opal is a sedimentary stone. In the time of formation of this stone, oozing water through the earth dissolved in silicates and become rich. The silicates are deposits as minute spheres when the water enters in a cavity.
The transparent spheres of amorphous silica and the voids between them form a 3D grating. This structural pattern is the reason of interference and diffraction of light from the surface or the interior of gems. This property helps to display multiple colors as the angle of incident light changes. The openings act as the diffracting gratings, which split the light into colors, and the layered structure creates interference. The passing light through the stone breaks into various colors. So, we can say diffraction of light take place by layers of silica spheres in its composition and it is responsible for the phenomenon play-of-color.
Shifting patches of spectral colors seen on the surface or in the interior of gem due to the phenomenon play-of-color. Reflected lights from different layers of silicon slightly overlap from sync waves and help certain spectral colors to decrease or remove and reinforce others. So, we get a group of shifting colors that have a trembling on and off effect. The patches of colors moved when the gem is turned around. The effect just like to a bubble with rainbow color, just more dramatic.
Large openings between the spheres do not allowed the light to bend when passing through them. That is why no diffraction takes place and we get common opal. Random shape and arrangement of spheres also produced common opal. Shape, size and regularity of the spheres influenced the colors seen. It also depends on the angle of view. Uniform shape and size of spheres diffract light and help to produce precious opal. On the other hand, smaller size of spheres helps to see more blue and violet colors. But the spheres must be smaller than 1500 angstroms. More reds and oranges are seen with bigger sized spheres. But the spheres should not be larger than 3500 angstroms.
Opal gets the name for its special optical phenomenon Play-of-color. The name came from Greek word ‘opallus’, which means to see a change in color. The quality of opal justified by the number of color exhibited by the gems. The most valuable opal is ‘black-opal’. Which has a strong play-of-color. Different types of colors flashed brilliantly by this stone. Due to dark background color it has given the name ‘black opal’. The colors of black opals are black, dark green, dark brown and others. These gems are transparent to translucent.
There can be seen many other opals, like white opal, crystal opal, fine opal, cherry, water opal and etc.
White opal is transparent to opaque. It has white background with Play-of-color. Crystal opal has no background color with strong play-of-color. This stone is transparent to translucent and lets the light pass through it unlike others. Fire opal is also transparent with yellow, orange, red or brown background color. It may have play-of-color or not. Fire opal with red background color is also known as cherry opal. Water opal is transparent to semitransparent with colorless background. It has little or Play-of-color.
The point of color seen in opal due to the Play-of-color is called as “fire”. The term ‘flash’ used to refer sudden play-of color as stone moved.
Gemologists divided the phenomenon play-of-color in various types, like pinfire, mosaic, flame, peacock etc. A minute, close-set flash of color is called as pinfire. A large, regular, angular, close-set flash of colors is known as mosaic. Sweeping reddish bands across the gem is called flame, whereas blue and green color effect is known as peacock.