An introduction to the formation, evaluation, and history of opals. Get to know how to choose and care for opals, and make sure you're aware of how to identify synthetics. Written by The Raw Stone's in-house gemologist, Rachel Kaminetsky.
History, background and formation of opal
The name “Opal” is derived from the original name the Romans gave this gemstone – Opalus, meaning “Precious Stone”. Opals are associated most commonly with the beautiful flashes of color they display, a phenomena called “Play-of-Color”, although not all opals display it. There are two broad categories of opal: precious opal, which display play-of-color, and common opal, which do not. Opal's internal structure is what will determine the occurrence of this phenomena.
Opal is a result of unique conditions: heavy seasonal rains that fall and drench the ground, in usually parched desert regions where the ground is rich in silica. Silica is is a compound of silicon and oxygen. When heavy rains fall, the water trickles down into the earth and carries silica-rich solutions into the cavities between the rocks. The hot summers dry the earth, and as the water evaporates the silica stays in place, and over millions of years, the opals form. Unlike gemstones such as quartz, opal does not have an atomic structure dictating its formation. At more than 30,000X, Australian scientists, John V. Sanders and Peter J. Darragh learned that opal is made up of billions of uniform, submicroscopic silica spheres, staked in a regular, three dimensional grid.
When light enters this uniform arrangement of silica spheres, it bends and diffracts, and produces play-of-color. When the silica spheres are of different shapes, sizes and of random arrangement, the light does not diffract, and common opal, with no phenomena, is the result.
When evaluating precious opal, three aspects are considered:
- Color - The background color of the gem, as well as the colors displayed in the play-of-color. In general, red is considered the best dominant hue in play-of-color, with orange next, followed by green. In addition, bright display of colors is preferred to faint colors. Background color refers to the color of the gem displaying the play-of-color.
- Pattern - Play of color occurs in one of three arrangements, called pinfire, flash, and harlequin. No matter the pattern, the colors must be bright, and pleasingly balance across the stone.
- Clarity - Clarity is its degree of transparency, as well as lack of inclusions such as fractures, surface blemishes and matrix.
Since opal is formed with the help of heavy rain, it contains a considerable amount of moisture. If an opal looses some of that moisture, it can develop a series of tiny fractures called crazing. Crazing not only detracts from an opals beauty, but also lessens it's durability. Crazing can be caused by exposure to excessive heat, dryness or direct light, and therefore should not be displayed in window displays and the like.
Types of opal
Opal types have been classified and reclassified for centuries, due to the gem's wide diversity. The jewelry industry has grouped opals into the following categories:
- Black opal - black body-color, shows play-of-color
- White opal - white body-color, shows play-of-color
- Crystal opal - background ranges from transparent to semi transparent and shows exceptional play-of-color
- Water opal - background ranges from transparent to translucent, may or may not show play-of-color
- Boulder opal - includes host rock or matrix in the finished gem, shows play-of-color
- Fire opal - backgrounds are reds, oranges or yellow, may or may not show play-of-color
- Assembled opal - layers of precious opal and other materials to improve beauty and/ or durability
An extra word on boulder opal
Boulder opal, as stated, is precious opal that was cut to include its host rock or matrix. The host rock is either sandstone or ironstone, and often serves as reenforcement for the less durable precious opal. This makes boulder opal more durable than other opal types. Bolder opal must be cut such that there is a balance between the play-of-color and the matrix. This usually dictates irregular cut shapes, making boulder opal ideal for one-of-a-kind jewelry designs.
Treatments and Synthetics
Although The Raw Stone offers natural, untreated gems only, it is important to know that opal treatments, synthesis and imitations are not uncommon. Opal treatments include sugar or smoke treatments to darken body-color and enhance play-of-color. Impregnation with wax, resin or polymers are used to fill pores and thus improve durability and beauty.
Opal imitations are made of plastic, glass, which can be identified by gas bubbles or inner “swirls”. Synthetic opal is made by creating the material and conditions for the formation of opal - in a lab. Under magnification synthetic opal display a structural pattern that looks like snakeskin, which tells gemologists that the stone is not natural.
Browse The Raw Stone's boulder opals here