Opal

In this episode of the I Can't Sleep Podcast, fall asleep learning about opals. You're probably thinking I'm going to read to you about gems and the like, but let's be honest: an opal is a rock. Yep, you're falling asleep learning about rocks! Happy sleeping.

Transcript

Welcome to the I Can't Sleep podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Boster.

Today's episode is from a Wikipedia article titled Opal.

Opal is a hydrated amorphous form of silica.

Its water content may range from 3 to 21% by weight,

But is usually between 6 and 10%.

Because of its amorphous character,

It is classed as a meteoroid,

Unlike crystalline forms of silica,

Which are classed as minerals.

It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock,

Being most commonly found with limonite,

Sandstone,

Rhyolite,

Marl,

And basalt.

Opal is the birthstone of October and the stone that celebrates the 14th anniversary of marriage.

The name opal is believed to be derived from the Sanskrit word opula,

Which means jewel,

And later the Greek derivative opalios,

Which means to see a change in color.

There are two broad classes of opal,

Precious and common.

Precious opal displays play of color.

Common opal does not.

Play of color is defined as a pseudo-chromatic optical effect,

Resulting in flashes of color light from certain minerals as they are turned in white light.

The internal structure of precious opal causes it to diffract light,

Resulting in play of color.

Depending on the conditions in which it formed,

Opal may be transparent,

Translucent,

Or opaque,

And the background color may be white,

Black,

Or nearly any color of the visible spectrum.

Black opal is considered rarest,

Whereas white,

Gray,

And green are the most common.

Precious opal.

Precious opal shows a variable interplay of internal colors,

And though it is a mineraloid,

It has an internal structure.

At microscopic scales,

Precious opal is composed of silica spheres,

Some 150 to 300 nanometers in diameter,

And a hexagonal or cubic,

Close-packed lattice.

It was shown by J.

V.

Sanders in the mid-1960s that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.

The regularity of the sizes and the packing of these spheres is a prime determinant of the quality of precious opal,

Where the distance between the regularly packed planes of spheres is around half the wavelength of a component of visible light.

The light of that wavelength may be subject to diffraction from the gradient created by the stacked planes.

The colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light.

The process can be described by Bragg's law of diffraction.

Visible light cannot pass through large thicknesses of the opal.

This is the basis of the optical band gap in the photonic crystal.

The notion that opals are photonic crystals for visible light was expressed in 1995 by Vazily Astrodov's group.

In addition,

Microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification.

The term opalescence is commonly used to describe this unique and beautiful phenomenon,

Which in gemology is termed play of color.

In gemology,

Opalescence is applied to the hazy milky turbid sheen of common or poach opal,

Which does not show a play of color.

Opalescence is a form of adulorescence.

For gemstone use,

Most opal is cut and polished to form a cabochon.

Solid opal refers to polished stones consisting wholly of precious opal.

Opals too thin to produce a solid may be combined with other materials to form attractive gems.

An opal doublet consists of a relatively thin layer of precious opal backed by a layer of dark colored material,

Most commonly ironstone,

Dark or black common opal,

Poach,

Onyx,

Or obsidian.

The darker backing emphasizes the play of color and results in a more attractive display than a lighter poach.

An opal triplet is similar to a doublet,

But has a third layer,

A domed cap of clear quartz or plastic on the top.

The cap takes a high polish and acts as a protective layer for the opal.

The top layer also acts as a magnifier to emphasize the play of color of the opal beneath,

Which is often an inferior specimen or an extremely thin section of precious opal.

Triplet opals tend to have a more artificial appearance and are not classed as precious gemstones.

Jewelry applications of precious opal can be somewhat limited by opal's sensitivity to heat,

Due primarily to its relatively high water content and predisposition to scratching.

Combined with modern techniques of polishing,

A doublet opal can produce a similar effect to solid black or bolder opal at a fraction of the price.

Doublet opal also has the added benefit of having genuine opal as the top visible and untouchable layer,

Unlike triplet opals.

Common Opal Besides the gemstone varieties that show a play of color,

The other kinds of common opal include the milk opal,

Milky bluish to greenish,

Which can sometimes be of gemstone quality,

Resin opal,

Which is honey yellow with a resinous luster,

Wood opal,

Which is caused by the replacement of the organic material in wood with opal,

Menelite,

Which is brown or gray,

Hyalite,

A colorless,

Glass-clear opal sometimes called Moller's glass,

Geyserite,

Also called salacious cinder,

Deposited around hot springs or geysers,

And diatomaceous earth,

The accumulations of diatom shells or tests.

Common opal often displays a hazy,

Milky turbid sheen from within the stone.

In gemology,

This optical effect is strictly defined as opalescence,

Which is a form of adulorescence.

Other Varieties of Opal Fire opal is a transparent to translucent opal with warm body colors of yellow to orange to red.

Although it does not usually show any play of color,

Occasionally a stone will exhibit bright green flashes.

The most famous source of fire opals is the state of Corredo Ro in Mexico.

These opals are commonly called Mexican fire opals.

Fire opals that do not show a play of color are sometimes referred to as jelly opals.

Mexican opals are sometimes cut in their rhyolitic host material if it is hard enough to allow cutting and polishing.

This type of Mexican opal is referred to as a cantera opal.

Also a type of opal from Mexico referred to as Mexican water opal is a colorless opal which exhibits either a bluish or golden internal sheen.

Gerasol opal is a term sometimes mistakenly and improperly used to refer to fire opals,

As well as a type of transparent to semi-transparent type milky quartz from Madagascar,

Which displays an asterism or star effect when cut properly.

However,

The true gerasol opal is a type of highlight opal that exhibits a bluish glow or sheen that follows the light source around.

It is not a play of color as seen in precious opal,

But rather an effect from microscopic inclusions.

It is also sometimes referred to as water opal too when it is from Mexico.

The two most notable locations of this type of opal are Oregon and Mexico.

Peruvian opal,

Also called blue opal,

Is a semi-opaque to opaque blue-green stone found in Peru,

Which is often cut to include the matrix in the more opaque stones.

It does not display a play of color.

Blue opal also comes from Oregon and Idaho in the Owyhee region,

As well as from Nevada around the Virgin Valley.

Opal is also formed by diadems.

Diodems are a form of algae that,

When they die,

Often form layers at the bottoms of lakes,

Bays,

Or oceans.

Their cell walls are made up of a hydrated silicon dioxide,

Which gives them structural coloration and therefore the appearance of tiny opals when viewed under a microscope.

These cell walls,

Or tests,

Form the grains for the diademaceous earth.

This sedimentary rock is white,

Opaque,

And chalky in texture.

Diodemite has multiple industrial uses,

Such as filtering or absorbing since it has a fine particle size and very porous nature,

And gardening to increase water absorption.

History Opal was rare and very valuable in antiquity.

In Europe,

It was a gem prized by royalty.

Until the opening of vast deposits in Australia in the 19th century,

The only known source was Servenica beyond the Roman frontier in Slovakia.

Opal is the national gemstone of Australia.

Because the primary source of opal are Australia and Ethiopia,

But because of inconsistent and widely varying accountings of their respective levels of extraction,

It is difficult to accurately state what proportion of the global supply of opal comes from either country.

Australian opal has been cited as accounting for 95 to 97 percent of the world's supply of precious opal,

With the state of South Australia accounting for 80 percent of the world's supply.

In 2012,

Ethiopian opal production was estimated to be 14,

000 kilograms by the United States Geological Survey.

USGS data from the same period,

2012,

Reveals that Australian opal production to be $41 million.

Because of the units of measurement,

It is not possible to directly compare Australian and Ethiopian opal production,

But these data and others suggest that the traditional percentages given for Australian opal production may be overstated.

Australia The town of Coober Pedy in South Australia is a major source of opal.

The world's largest and most valuable gem,

Opal Olympic Australis,

Was found in August 1956 at the 8-mile opal field in Coober Pedy.

It weighs 17,

000 carats and is 11 inches long,

With a height of 4 and three-quarter inches and a width of 4 and a half inches.

The Mentabi Opal Field,

Located about 250 kilometers northwest of Coober Pedy,

Has also produced large quantities of crystal opal and the rarer black opal.

Over the years it has been sold overseas incorrectly as Coober Pedy Opal.

The black opal is said to be some of the best examples found in Australia.

Andamica in South Australia is also a major producer of Matrix Opal,

Crystal Opal,

And Black Opal.

Boulder Australian Town,

Lightning Ridge in New South Wales,

Is the main source of black opal,

Opal containing a predominantly dark background,

Dark gray to blue-black displaying the play of color.

Collected from the Greeman Creek Formation,

Boulder Opal consists of concretions and fracture fillings in a dark,

Salacious,

Ironstone matrix.

It is found sporadically in western Queensland from Kainuna in the north to Yawa and Koroit in the south.

Its largest quantities are found around Junda and Quilpe in southwest Queensland.

Australia also has opalized fossil remains,

Including dinosaur bones in New South Wales in South Australia and marine creatures in South Australia.

Ethiopia It has been reported that Northern Africa opal was used to make tools as early as 4000 BC.

The first published report of gem opal from Ethiopia appeared in 1994 with the discovery of precious opal in the Men's Gish District,

North Sherwe Province.

The opal,

Found mostly in the form of nodules,

Was of volcanic origin and was found predominantly within weathered layers of rhyolite.

The Sherwe Province opal was mostly dark brown in color and had a tendency to crack.

These qualities made it unpopular in the gem trade.

In 2008,

A new opal deposit was found approximately 180 kilometers northwest of Sherwe Province,

Near the town of Wejotena in Ethiopia's Wolo Province.

The Wolo Province opal was different from the previous Ethiopian opal finds in that it more closely resembled the sedimentary opals of Australia and Brazil,

With a light background and often vivid play of color.

Wolo Province opal,

More commonly referred to as Welo or Welo opal,

Has become the dominant Ethiopian opal in the gem trade.

Virgin Valley,

Nevada The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black,

Crystal,

White,

Fire,

And lemon opal.

The black fire opal is the official gemstone of Nevada.

Most of the precious opal is partial wood replacement.

The precious opal is hosted and found in situ within a subsurface horizon or zone of bentonite,

Which is considered a load deposit.

Opals which have weathered out in the in-situ deposits are alluvial and considered placer deposits.

Miocene age opalized teeth,

Bones,

Fish,

And a snake head have been found.

Some of the opal has high water content and may desiccate and crack when dried.

The largest producing mines of Virgin Valley have been the famous Rainbow Ridge,

Royal Peacock,

Bonanza,

Opal Queen,

And WRT Shone Tree Black Beauty Mines.

The largest unpolished black opal in the Smithsonian Institution,

Known as the Roebling Opal,

Came out of the tunneled portion of the Rainbow Ridge Mine in 1917 and weighs 2,

585 carats.

The largest polished black opal in the Smithsonian Institution comes from the Royal Peacock Opal Mine in the Virgin Valley,

Weighing 160 carats,

Known as the Black Peacock.

Mexico Opal occurs in significant quantity and variety in central Mexico,

Where mining and production first originated in the state of Coetero.

In this region the opal deposits are located mainly in the mountain ranges of three municipalities,

Colon,

Tequiscupan,

And Ezequiel Montes.

During the 1960s through the mid-1970s,

The Coetero mines were heavily mined.

Today's opal miners report that it was much easier to find quality opals with a lot of fire and play of color back then,

Whereas today the gem quality opals are very hard to come by and command hundreds of US dollars or more.

The oldest mine in Coetero is in Santa Maria del Eris.

This mine was opened around 1870 and has been reopened at least 28 times since.

At the moment there are about 100 mines in the regions around Coetero,

But most of them are now closed.

The best quality of opals came from the mine Santa Maria del Eris,

Followed by La Hacienda La Esperanza,

Fuentesuelas,

La Carbonera,

And La Trinidad.

Important deposits in the state of Jalisco were not discovered until the late 1950s.

In 1957,

Alfonso Ramirez of Coetero accidentally discovered the first opal mine in Jalisco.

La Unica located on the outer area of the volcano of Tequila,

Near the Juzquilipan farm in Magdalena.

By 1960 there were about 500 known opal mines in this region alone.

Other regions of the country that also produce opals of lesser quality are Cuerero,

Which produces an opaque opal similar to the opals from Australia.

Some of these opals are carefully treated with heat to improve their colors,

So high quality opals from this area may be suspect.

There are also some small opal mines in Morelos,

Durango,

Chihuahua,

Baja California,

Guanajuato,

Pueblo,

Mijo Juan,

And Estado de Mexico.

Other Locations Another source of white base opal or creamy opal in the United States is Spencer,

Idaho.

A high percentage of the opal found there occurs in thin layers.

Other significant deposits of precious opal around the world can be found in the Czech Republic,

Canada,

Slovakia,

Hungary,

Turkey,

Indonesia,

Brazil,

Honduras,

Guatemala,

And Nicaragua.

In late 2008 NASA announced it had discovered opal deposits on Mars.

Synthetic Opal Opals of all varieties have been synthesized experimentally and commercially.

The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974.

The resulting material is distinguishable from natural opal by its regularity.

Under magnification,

The patches of color are seen to be arranged in a lizard skin or chicken wire pattern.

Furthermore,

Synthetic opals do not fluoresce under ultraviolet light.

Synthetics are also generally lower in density and are often highly porous.

Opals which have been created in a laboratory are often termed lab-created opals,

Which,

While classifiable as man-made and synthetic,

Are very different from their resin-based counterparts which are also considered man-made and synthetic.

The term synthetic implies that a stone has been created to be chemically and structurally indistinguishable from a genuine one,

And genuine opal contains no resins or polymers.

The finest modern lab-created opals do not exhibit the lizard skin or columnar pattern of earlier lab-created varieties,

And their patterns are non-directional.

They can still be distinguished from genuine opals,

However,

By their lack of inclusions and the absence of any surrounding non-opal matrix.

While many genuine opals are cut and polished without a matrix,

The presence of irregularities in their play of color continues to mark them as distinct from even the best lab-created synthetics.

Often research in macroporous structures have yielded highly ordered materials that have similar optical properties to opals and have been used in cosmetics.

Synthetic opals are also deeply investigated into photonics for sensing and light management purposes.

Local Atomic Structure of Opals The lattice of spheres of opal that cause interference with light is several hundred times larger than the fundamental structure of crystalline silica.

As a mineraloid,

No unit cell describes the structure of opal.

Nevertheless,

Opals can be roughly divided into those that show no signs of crystalline order,

Amorphous opal,

And those that show signs of the beginning of crystalline order,

Commonly termed cryptocrystalline or microcrystalline opal.

Opality The word opal is adapted from the Latin term opalis.

The origin of this word,

In turn,

Is a matter of debate,

But most modern references suggest it is adapted from the Sanskrit word upala.

References to the gem are made by Pliny the Elder.

It is suggested to have been adapted from Ops,

The wife of Saturn,

And goddess of fertility.

The portion of Saturnalia is devoted to Ops was opolia,

Similar to opalis.

Another common claim that the term is adapted from the ancient Greek word apolios.

This word has two meanings.

One is related to seeing,

And forms the basis of the English words like opaque.

The other is other,

As in alias and altar.

It is claimed that apolios combined these uses,

Meaning to see a change in color.

However,

Historians have noted the first appearances of apolios do not occur until after the Romans had taken over the Greek states in 100 BC,

And they had previously used the term pederos.

However,

The argument for the Sanskrit origin is strong.

The term first appears in Roman references around 250 BC,

At a time when the opal was valued above all other gems.

The opals were supplied by traders from Bosporus,

Who claimed the gems were being supplied from India.

Before this,

The stone was referred to by a variety of names,

But these fell from use after 250 BC.

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Very interesting to learn more about my birthstone. Thank you for sharing this with us here as part of your excellent series of sleepy talks. I see you and the light within you. Be well. 🤲🏻💖🤲🏻

Brilliant

Thank you Benjamin, for your non stop, very interesting, bore me to sleep readings. Opals are my favorite stones so I will listen to this often, see if I can listen long enough to learn.

Wonderful as always! Put me right to sleep. Thank you! 🤗🤗🤗