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Opal Gemstone

Opal is naturally found in a white coloured semi-precious gemstone composed of a silicate mineral family recognized for its marvellous ‘play of colours’. This astrological alternative of precious diamond is worn for gaining success in innovative pursuits, lavish lifestyle, social/financial prominence, matrimonial harmony and good health. Over thousands of years, people have quarried and treasured opals. These striking opal stones have energized a rich body of folklore. (Thus, opal gemstones have been considered both the luckiest and unluckiest stones one can wear).


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Factual Information About Opal Gemstone

Name

Opal Stone

Varieties/ Different Names

Agate Opal, Andean Opal, Black Opal, Boulder Opal, Common Opal, Crystal Opal, Cacholong Opal, Girasol, Fire Opal, Honey Opal, Hyalite, Hydrophane, Harlequin Opal, Jasper Opal, Matrix Opal, Moss Opal, Milk Opal, Pink Opal, Prase Opal, Precious Opal,  Semiopal,  Water Opal, White Opal Wax Opal, Wood Opal

Birthstone

October

colour

colourless, white, red (various shades), orange, yellow, yellowish brown, blue, black, grey, green, and violet.

Chemical Formula

SiO2 · nH2O. Water= 1-21% in opal, generally 6-10% in precious opal.

Refractive Index

1.37-1.47

Hardness

5.5-6.5

Luster

Vitreous, waxy, pearly.

Polish Luster

Vitreous to resinous

Fracture Luster

Waxy, sub vitreous

Fracture

Conchoidal to uneven

Wearability

Poor

Transparency

Transparent to opaque

Luminescence

Yes

Luminescence Present

Fluorescent, Phosphorescent, UV-Short, UV-Long. Green fluorescence in opal commonly because of included U minerals. Much opal fluoresces strong white in SW, LW, with persistent phosphorescence

Enhancements/ Treatment

Dyeing, Surface Coating, Infusion/Impregnation,

Care Instructions

Heat sensitive, clean with room temperature soap and water. Avoid wearing gemstone or opal jewellery where it will get rough treatment.

Occurrence

In sedimentary rocks or where low temperature solutions carrying silica can seep through rocks


Assembled Opal Gems

A triplet from assembled opal gems adds a transparent quartz cap and makes for a  good opal stone ring and opal earrings, since the hard quartz keeps the softer opal from scratching.


How to Determine Synthetic Opals?

Synthetic or lab-created opals are actually the real opals, but they’re grown in laboratories rather than yellowish-brown underground. They encounter the same formation processes, only at a stimulated rate in controlled settings.

Synthetic opals may exhibit an intense display of colour, generally in a mosaic pattern. With high magnification and backlighting, you can find a scale-like, snake skin or chicken wire structure in the pattern. When put under high magnification with transmitted light, synthetics may display a dendritic structure. Synthetic opals don’t phosphoresce (sparkle or twinkle) and may also stick to the tongue.


How to Detect Fake or Imitation Opal Gems?

Contrary to lab-created opals, imitations only simulate the physical look of opals. These imitations are essentially made from glass or plastic.

Plastic imitations or stimulants are soft and can be dissected with a sharp needle. They do not sparkle or twinkle (phosphoresce).

Glass stimulants essentially consist of glass bubbles and swirl marks. They also do not phosphoresce. Their refractive index (RI) and specific gravity (SG) are also generally higher than natural opal.


Care For Opal Stones

Nowadays, used frequently as opal necklace, opal rings and other opal jewellery, extra care of the delicate gemstone is well worth it. Highly sensitive to changes in temperature as well as a “crazing”, that implies they can easily form cracks or “craze” as they dehydrate, and must be kept cautiously.

Opals kept in water must be dried properly and cautiously before cutting.

Sometimes opals in rings can become chalky white and weary. This may owe to a network of scratches on the opal surface that dismantle the polish and dulls the colour play. However, a simple re-polishing can generally correct this. Having a hardness of only 5.5 makes them quite susceptible to scratching. Opals are thus usually not recommended for ring stones, unless the stone is positioned in a protective setting or a triplet for occasional wear.


Opal Stone Facts and Information

  • Opal stone is sedimentary.

  • Many ancient references to opal may actually indicate other gems, such as the iridescent iris agate.

  • Under proper conditions, water seeps through the earth, becoming affluent in dissolved silicates. When water makes way through a cavity, it accumulates the silicates as microscopic spheres, forming opals.

FAQs (Frequently Asked Questions)

1. How to Differentiate Between “Common Opals” and “Precious Opals”?

Answer:  Opal is an amorphous type of silica, chemically identical to quartz, but consisting of 3% to 21% water within its mineral composition. Gem grade opals generally contain 6% to 10% water content.


With respect to “Common Opals” and “Precious Opals”— spheres that are uniform in shape and size are neatly stacked, and will diffract light. These stones are known as precious opals. Conversely, if the spheres are random in shape, size and arrangement, the outcome is common opals.


Common opals can display an opaque or glassy appearance having a waxy luster. Seldom cut, these stones come in a huge variety of colours. Common opals are commonly fluorescent. On the other hand, Precious opals, also referred to as “noble opals,” exhibit fire or play of colours.

2. What is Meant By “Play of Colours”?

Answer: Opal’s play of colours or characteristic fire was long believed to be an outcome of iridescence. However, with the emergence of scanning electron microscopes, we now know it’s an outcome of diffraction. This mechanism of flashing or moving colours due to diffraction isn’t linked to the body colour of the opal.


Specific colours observed in an opal’s fire depend on the size of the spheres and the angle of viewing. For example, black opal obtains its colour from volcanic ash, but inclusions are unessential to the play of colour. That is totally because of the tiny silicate spheres. For blue and violet colours, they must be smaller than 1,500 angstroms (Å), but no bigger than 3,500 Å to generate reds and oranges.

3. What Does “Fire” in Opal Mean?

Answer: the term “fire” essentially refers to a gemstone’s dispersion. While it’s admissible to imply to opal’s play of colour as fire, it’s not acceptable to suggest gemstone dispersion as “play of fire.” Only opals display a play of colour.

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