Origin and Properties of Precious Opals

Opals have amazed us for ages with their magical play of color. 

But what are these rainbow gems made of, and where do they come from? Let's see the science behind precious opals.

 What Are Opals?

Chemically speaking, opals are a hydrated, amorphous form of silica - aka silicon dioxide or SiO2. Amorphous means opals lack a defined crystalline structure. Irregularly arranged silica molecules in opals make up this stunning crystal.  

This random molecular layout gives opals their soft, waxy texture. It also makes them a mineraloid rather than a traditional mineral. Minerals have orderly crystalline patterns, while mineraloids like opals have no distinct crystal shape. 

Opals contain variable amounts of water, from 3% to 21% by weight. The water slides into gaps and openings in the loose silica structure. And as we'll see, this trapped water is vital to opal's magical properties. 

Where Do Opals Come From?

Unlike minerals that crystallize within rigid conditions, opals form freely. They originate relatively close to the earth's surface in rock fractures and cavities.

 As silica-rich groundwater flows through these cracks and voids, bits of silica gradually accumulate. The silica precipitates out of solution and deposits along the rock walls in scattered, haphazard orientations. 

Over hundreds and thousands of years, steady silica buildup leads to opal formation. Commonly, cavities within sedimentary rocks like sandstone or basalt line with opal deposits. They grow in the path of slowly seeping, mineral-rich waters.

 Precious vs. Common Opal

There are two varieties of opal: precious and common. The critical difference lies in the play of color.

 Precious opal demonstrates a play of color - flashes of spectral color - when turned in light. This rainbow sparkle arises from the internal structure.

 Common opal lacks a play of color and appears milky or turbid. While lovely, common opal doesn't command the value of precious play-of-color opal.

 Why Does Precious Opal Sparkle?

What gives Precious Opal its spellbinding play of color? It comes down to the uniform silica spheres within the loose opal structure.

 These tiny silica particles stack in an orderly grid-like pattern, like a crystal lattice. But unlike a continuous crystal, the spheres have gaps between their stacked layers.

 The water-filled spaces act as transmission channels when light hits this layered silica grid. Some wavelengths move through unimpeded, while other colors diffract and separate into the spectrum.

 As light fluctuates through this silica network, it disperses into shimmering flashes of changing color. Different silica layers and paths split the light when turning an opal to generate a dynamic rainbow display.

 The precise size and uniformity of the stacked silica spheres determine the quality of play of color. Expert cutters orient opals to maximize the diffracted spectrum display.

 The Rarest and Most Common Opals

Opals come in all colors of the rainbow, but some varieties are way more prized than others:

• Black opals - These crazy rare beauties show off shimmering colors against a dark body. They come from Lightning Ridge in New South Wales, Australia.

• White or Light opals - In this type of opal, the Play-of-color flashes against a white background. These glimmers originate from Coober Pedy in South Australia.

• Boulder opals - Formed inside ironstone boulders, these opals have vivid color but thin layers. This opal is found in Queensland, Australia.

• Crystal or Water opals - This opal is Translucent with a bluish body from Mexico and the US. 

More common opals include milky darker European stones and white clay opals from Peru. They're abundant but lack the most desired play-of-color.

 What Makes Them Shiny?

The incredible luminous quality comes from light bouncing off the internal silica layers. The beams reflect and scatter, creating a gentle, milky iridescence.

 This opalescence adds to the delicate, mystical vibe of the gem.

 When worn as jewelry, it casts a flattering, warm glow on the skin. The color flashes combine to make it no wonder that opals have been treasured for ages. Their visual magic is unmatched in the gem world. 

The Opal's Symbolic Meaning

With their rainbow shine, opals have carried symbolic meaning in cultures across the world:

• Hope - Their rare beauty makes them a symbol of hope.

• Purity - The prismatic flashes evoke innocence and purity.

• Love - Exchanging opal jewelry can signal a deep emotional connection.

• Creativity - The random colors inspire artists and dreamers.

 According to Arabic folk tales, opals formed from lightning strikes sparking colorful streaks into desert sands. It must be scientifically accurate but beautifully captures opals' fiery, fleeting allure. 

Like lightning, opals shine momentarily, then disappear into the imagination. 

Their ephemeral rainbows continue to captivate centuries on. No gem has quite the same magical charm.