Opal Description

"The Path of Enlightenment" necklace contains 180 magnificent opals from Lightning Ridge, Australia, a famous opal producing area. - Courtesy Cody Opal

Opal is the product of seasonal rains that drenched dry ground in regions such as Australia’s semi-desert “outback.” The showers soaked deep into ancient underground rock, carrying dissolved silica (a compound of silicon and oxygen) downward.

During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal.

How Opal Forms
Opal is known for its unique display of flashing rainbow colors called play-of-color. There are two broad classes of opal: precious and common. Precious opal displays play-of-color, common opal does not.

Play-of-color occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of Ping-Pong balls in a box. As the lightwaves travel between the spheres, the waves diffract, or bend. As they bend, they break up into the colors of the rainbow, called spectral colors. Play-of-color is the result.

This black opal exhibits exceptional play-of-color. - Courtesy Cody Opal

The color you see varies with the sizes of the spheres. Spheres that are approximately 0.1 micron (one ten-millionth of a meter) in diameter produce violet. Spheres about 0.2 microns in size produce red.  Sizes in between produce the remaining rainbow colors.

This fine-quality light opal is from Mintabie, South Australia. Almost all of its spectral colors are visible from a variety of viewing angles. - Courtesy Cody Opal

Although experts divide gem opals into many different categories, five of the main types are:

• White or light opal: Translucent to semitranslucent, with play-of-color against a white or light gray background color, called bodycolor.
• Black opal: Translucent to opaque, with play-of-color against a black or other dark background.
• Fire opal: Transparent to translucent, with brown, yellow, orange, or red bodycolor. This material—which often doesn’t show play-of-color—is also known as “Mexican opal.”
• Boulder opal: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
• Crystal or water opal: Transparent to semitransparent, with a clear background. This type shows exceptional play-of-color.

This exceptional light opal is from South Australia’s Coober Pedy opal fields. - Courtesy Cody Opal

The rough specimen is a thin seam of boulder opal on sandstone host rock. The 3.47-carat boulder opal tablet was cut from a similar piece of rough.

0.2 MICRONS

Grids of silica spheres 0.2 microns in size create red play-of-color flashes.

20% WATER

Opal contains up to 20% water trapped in its silica structure.
 

1829

The novel “Anne of Geierstein” gave opal a reputation of being unlucky.

FACTS

• MINERAL: Hydrated Silica
• CHEMISTRY: SiO₂•nH₂O
• COLOR: All colors
• REFRACTIVE INDEX: 1.37-1.47
• BIREFRINGENCE: None
• SPECIFIC GRAVITY: 2.15 (+0.08, -0.90)
• MOHS HARDNESS: 5 to 6.5
• TREATMENTS
  There are a number of processes used to alter the color, apparent clarity, or improve the durability of gems.
  
  
  SYNTHETICS
  Some gemstones have synthetic counterparts that have essentially the same chemical, physical, and optical properties, but are grown by man in a laboratory.
  
  
  IMITATIONS
  Any gem can be imitated—sometimes by manmade materials or by natural materials chosen by man to impersonate a particular gem.

  QUALITY FACTORS

  Play-of-color, intensity, and pattern are important value factors.
  • COLOR
    
    Opal’s spectacular play-of-color can display all the colors of the rainbow.
  • CLARITY
    
    Experts expect different levels of clarity for different types of opals.
  • CUT
    
    Fine opals are often cut into irregular shapes that keep as much play-of-color as possible.
  • CARAT WEIGHT
    
    Opal has relatively low density so even larger sizes can be comfortable to wear.