The many legends and tales surrounding the opal can be traced back to the colorful stone’s origin, thousands of years ago in the Australian mainland.In some of these legends, it is said that the creator came down to Earth on a rainbow bridge in order to bring the message of peace to all humans. At the spot where his foot first touched the ground, the stones came alive and sparkled in all the colors of the rainbow. That was the birth of the opal.

 

Scientifically, opal is known as SiO2·nH2O - an amorphous mineraloid comprised of hydrated silicon dioxide (a mix of silica and water). The opal's appearance can range from colorless, white, or light grey to dark grey or black. However the truly unique thing about opal is that it displays all the colors of the spectrum in a play of color, a result of the interference and diffraction of light passing through tiny silica spheres in the microstructure of the opal.

 

Precious opal is defined as opaline silica with a play of color. The term play of color was originally created to describe the beautiful shifting of opal's spectral hues. Australian opal is referred to as sedimentary opal because it is found primarily in the Mesozoic Great Artesian Basin’s sedimentary rock. Australian precious opals usually contain around 5-6% water and consist of small silica spheres arranged in a regular pattern. Its hardness of 5.5 to 6.5 on the Mohs scale puts it about halfway down the range (roughly the same hardness as glass). It has a specific gravity of 1.9 to 2.3, depending on the amount of water present.

 

Opal is formed from a solution of silicon dioxide and water. As water seeps through sandstone, it picks up tiny particles of silica. Millions of years ago, the solution flowed into cracks and voids in the sediment as well as the volcanic areas of inland Australia. Estimates suggest this solution had a deposition rate of approximately one centimetre thickness every five million years at a depth of forty meters.

 

As the silica in solution was deposited and the water content gradually decreased, spheres formed in the gel. The spheres are formed by the particles of silica spontaneously adhering to other particles that form around them. These spheres of amorphous silica range in size from 1500 to 3500 angstroms (1 angstrom is 1 ten millionth of 1 millimeter).

 

The spheres are not only remarkably uniform in size but, in gem quality opal, are packed in a very regular array. Because they are spherical, there are tiny holes remaining in the structure (much the same as when marbles are placed together in a container) and these holes too are arranged in a regular three-dimensional way. The appearance of an opal is an optical diffraction of visible light because of the arrays of these cavities..   

 

When the spheres are bigger (about 3500 angstroms in diameter) the red and orange colors are produced. At the other end of the scale, at about 1500 angstroms in diameter, the blue end of the spectrum is diffracted. Between these figures occur the rest of the rainbow colors.  From this it can be deduced that the light diffraction in the voids is greatest when the sphere size is greatest. Thus, red is usually the brightest color and the blue duller.

 

In summary, the color in precious opal is caused by the regular array of silica spheres and voids diffracting white light and breaking it into the colors of the spectrum. The diameter and spacing of the spheres controls the color range of an opal. Small spheres produce opal of blue color only (the most common), whereas larger spheres produce red (the rarest color).