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What color is opal and tourmaline?

By: Author Color With Leo

What color is opal and tourmaline?

Opal and tourmaline are two popular gemstones known for their beautiful colors and intricate patterns. Though they share some similarities, opal and tourmaline have distinct properties when it comes to their coloration. Understanding the cause of color in these gems can help jewelers, collectors, and gem enthusiasts appreciate their unique beauty.

What Causes Color in Opal?

Opal is a hydrous silicon dioxide mineraloid with water content typically between 3-21%. The water is present in the form of silica spheres or cavities within the opal structure. When white light enters opal, the silica spheres and cavities refract and diffract the light to produce flashes of spectral color. This phenomenon is known as opalescence.

The exact color observed depends on the size and uniformity of the silica spheres. Smaller, more uniform spheres tend to produce blue and green colors. Larger, less uniform spheres produce warmer reds and oranges. The background body color also impacts the observed play-of-color. Darker body color opal mute the play-of-color, while lighter body colors intensify it.

There are two main varieties of opal based on their play-of-color:

Precious Opal

Precious opal shows a vivid play-of-color and is classified into two main types:

  • White opal – Transparent to translucent opal with play-of-color against a white or light body color
  • Black opal – Translucent to opaque opal with play-of-color against a black or dark body color

Common Opal

Common opal does not display play-of-color and has a milky white, yellow, red, grey, or brown body color. The color results from impurities within the opal structure. Iron oxides produce yellows and reds, while organic materials produce grey and brown hues.

What Causes Color in Tourmaline?

Tourmaline is a crystalline boron silicate mineral. Its diversity of color stems from its complex chemical composition and structure. The general chemical formula for tourmaline is:

XY3Z6(T6O18)(BO3)3V3W

Where:

X = Ca, Na, K, ▢ (vacancy)
Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+
Z = Mg, Al, Fe3+, Mn3+, V3+, Cr3+, Ti4+
T = Si, Al, B
B = B
V = OH, O
W = OH, F, O

The wide range of possible ion substitutions makes tourmaline an extremely complex mineral. The transition metal cations (like iron, chromium, manganese, titanium) are mainly responsible for tourmaline’s coloration. Different chemical compositions produce tourmalines in every color of the rainbow. The main varieties include:

  • Red – Caused by iron ions. Shade depends on whether Fe2+ or Fe3+ ions dominate.
  • Blue – Caused by iron and titanium ions.
  • Green – Caused by chromium, iron, and/or vanadium ions.
  • Pink – Caused by manganese ions.
  • Yellow – Caused by iron ions.
  • Brown – Caused by iron and manganese ions.
  • Black – Caused by iron, manganese, and titanium ions.
  • Colorless – Complete absence of transition metal ions.

In addition to its chemical composition, tourmaline’s color depends on its crystal structure. The mineral commonly occurs as long prismatic crystals displaying strong pleochroism. Pleochroism is the ability to display different colors when viewed along different crystal axes. This adds to the variety of hues and multicolored zones within tourmaline crystals.

Comparative Colors of Opal and Tourmaline

While opal and tourmaline can both display a myriad of colors, there are some key differences:

  • Opal’s colors come from the diffraction of white light, producing spectral flashes. Tourmaline’s colors stem from its complex chemical composition.
  • Opal’s body color impacts the observed play-of-color. Tourmaline’s pleochroism allows it to display multiple colors in one crystal.
  • Opal only displays play-of-color when it has 3-21% water. Dehydration causes opal to lose its play-of-color permanently. Tourmaline’s color is fixed and generally stable.
  • The most desired and valuable opal displays every color of the spectrum. Each tourmaline variety has its own distinctive color caused by specific chemical components.

Notable Stones

Some exceptional examples of opals and tourmalines include:

Opals

  • The Olympic Australis Opal – The largest and most valuable opal ever found at 11,000 carats. Discovered in 1956 at Eight Mile opal field, Coober Pedy, Australia. Displayed at the Melbourne Museum, Australia.
  • The Andamooka Opal – Discovered in 1949 in South Australia. Weighs 203.84 carats with a diverse play-of-color. Displayed at the South Australian Museum.
  • The Flame Queen Opal – Discovered in 1938 at Tarcoola, South Australia. Weighs 433 carats with a predominance of red play-of-color. Displayed at the Royal Ontario Museum, Canada.

Tourmalines

  • The Watermelon Tourmaline – A rare bi-color pink and green tourmaline from Mogok, Myanmar. Weighs 48.93 carats and cut en cabochon. Displayed at the Smithsonian Museum of Natural History.
  • The Blue Heart – The world’s largest cut blue tourmaline at 30.82 carats. Mined in 1989 in Brazil. Currently in a private collection.
  • The Ertl Frog – Carved black tourmaline sculpture weighing 2,290 carats. Mined in San Diego County, California in 1898. Displayed at the Houston Museum of Natural Science.

These exceptional gems demonstrate the diverse color potential of opal and tourmaline.

Conclusion

In summary, the color of opal stems from the diffraction of light off its silica spheres and cavities. Tourmaline’s wide array of hues originate from its complex chemical composition and crystal structure. Though they share an ability to display many colors, their color mechanisms differ substantially. Understanding the science behind opal and tourmaline’s colors allows for greater appreciation of their uniqueness and beauty as gemstones. With proper care, their mesmerizing colors will continue amazing gem aficionados for generations to come.