Mineral Mondays #4: Opal

in #science7 years ago (edited)

Opal

Opal is a hydrated amorphous silica, meaning it's silica that has water in it, a lot, between 3-21% water content. It forms in cracks/fissures in rock as water and silica fill in those spaces over time and harden.

There are two types of opal, precious (has fire) & common(no fire). Precious opal gets it's beautiful color from its microscopic, crystal, sphere structure that diffracts, bends, light. This characteristic is called "Play of Colors" and it is unique to precious opal. It can produce every prismatic color from violet to red. Precious opal usually comes in white or black and common opal comes in many other colors like blue, brown, pink and orange to name a few. Let's have a look at a couple of precious opals and a common opal.

Precious Opal

Both are precious opals and show a wide range of color, however the set opal has much more vivd color. It is also more translucent than the second opal making it much more valuable. While the second precious opal has beautiful fire, it is more opaque and the fire is much shorter in length. It is still a valuable precious opal, just not a top grade precious opal.

Blue Common Opal from Japan

While the common opal is pretty, it lacks the fire that makes opal so special. It's more of a collector's mineraloid

Some opal facts:

  • The Opal family - precious opal, common opal, fire opal, girasol opal and Peruvian opal (blue opal)
  • Shape - irregular
  • Composition - silica & water
  • Hardness - 5.5 - 6 (middle, diamond is 10)
  • Refractive index (fire) - 1.37 - 1.47 (diamond is 2.417 - 2.419)
  • October Birthstone (shared with Tourmaline)

Boulder Opal

My favorite kind of opal is boulder opal from Australia. Side note, Australia produces something like 95% of the world's opals. Boulder opal is an opal that has formed in cracks in ironstone. Ironstone is a hard, dark brown mineral containing iron ore that can take a polish making it usable in jewelry. Below are pictures of some of our boulder opals. Enjoy!

Opal is found all over the world, but there are a few places that the fantastic, gem grade material comes from:

  1. Australia - There are famous opal fields in western Australia's Queensland like Yowa & Koroit and South Austrailia's Coober Pedy.
  2. Ethiopia - Wallo aka Wello
  3. Nevada - Virgin Valley

Here are some specimens from localities outside of Australia.

Spencer, Idaho Opal

Ethopian Opal

Ice Cream Opal from California

Pink Common Opal from Peru

Blue Common Opal from Oregon

We've been collecting opal for several years and my wife immediately took to the material, using it in her jewelry. Her jewelry can be found at our Japan site: https://www.rt395mineralsjp.com .

Some of My Wife's Beautiful Precious Opal Earrings

In Closing Here Are Some More Opal Fun Facts:

  • Did you know that in 2008 NASA discovered Opal on Mars!? Even I didn't know this until researching this post. One can only imagine the treasures that Mars holds for us rockhounds.
  • The most valuable opal in the world is the Virgin Rainbow, valued at over $1,000,000.
  • The largest opal found is the Olympic Australis weighing 17,000 carats, 3450 grams.

Next Mineral Monday #5: Pentagonite


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    Drooling over your awesome opals!
    I've also wondered what rocks are like on other planets, the cave of giant crystals in Naica Mexico was mindblowing to see the shows on that, is like another world. I look forward to seeing more of your rocks, I have loved rawks since I could first pick one up, can't have just one lol

    Thank you. Indeed, you can only imagine what is out there on other planets regarding minerals.

    The giant gypsum crystals in Naica were unbelievable and deadly. It's great that they documented them and then decided to seal the cave back up so it could fill up with water again.

    If you have rocks share them on steemit!

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