Detailed Analysis of Opal Gemstone: Structure, Colors, and Uses

This report provides a comprehensive overview of opal gemstones, starting with an introduction to their composition as amorphous, hydrated silica. It details the different types of opal, including common, fire, and precious opal, emphasizing the unique "play of color" phenomenon exhibited by the latter. The report explains the color-producing mechanism, which involves the diffraction of light within the gemstone's uniformly arranged microscopic spheres. It also discusses how sphere size and arrangement determine the colors produced. The report further explores the various uses of opal gemstones, particularly in jewelry and in therapeutic practices, highlighting the beliefs associated with different opal colors. In conclusion, the report summarizes the key characteristics of opal gemstones, emphasizing their value and diverse applications. The report also includes references to several scientific papers and websites that support the information.

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Opal Gemstone
Introduction and General Overview
Opal gemstone is an amorphous and hydrated form silica with water content ranging between 3
and 21 percent of the mineraloid's weight. The hydrous silicon dioxide (SiO2nH2O) lacks
definite chemical composition and crystalline structure unlike other compounds of silica (Laurs
et al. 383). Due to the absence of crystalline structure, it is referred to as a mineraloid rather than
a mineral. The lack of crystals in its internal structure enables opal to diffract light making it
flash all the colors of the spectrum (Floter et al. 4). As one of the most valuable types of
gemstone, the best quality opal is equally as expensive as rubies, emeralds, and diamonds.
Opal is commonly known for its colors. Its value depends on the various colors it
produces. Its beauty makes it suitable for decorating jewelry such as earrings, some scuffs, and
brooches. The most familiar type of opal is "potch" frequently called "common opal," and makes
up approximately 90 percent of this type of gemstone ("What is Diffraction"). Its value is
considerably low compared to other types of opal since it has pearly luster with a milky
appearance. Regardless, by polishing, specimens of common opal are lustrous and attractive. The
second type of opal is "fire opal" which is either transparent or translucent with yellow, orange,
or red color at the background. Finally, precious opal is the most valuable and a very rare
gemstone (Laurs et al. 395). It exhibits a phenomenon called "play of colors" which is the
flashing of bright colors upon exposure to or removal from a source of light.

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The Color Producing Mechanism
Some rare types of opal flash bright colors when turned towards the light. The flashing of colors
is called play of color. Precious opals are the only specimen of this type of gemstone that
exhibits this quality. Mineralogists have studied this trait and identified that it occurs due to
diffraction of light within the uniformly distributed spheres inside the mineraloid (Landgrebe et
al. 78). Some of the conditions that lead to the observation of the play of color are moving the
stone towards the light source, removing the source of light away from the stone, or variation of
the angle of viewing the stone. Therefore, the motion is a crucial requirement for noticing the
play of color.
It is essential to study the nature of spheres within the opal gemstone and determine why
the occurrence is only possible for the precious opal. As mentioned earlier, opal is a hydrated
compound of silicon dioxide. It lacks crystals but consists of spheres that determine its structure.
During rainy seasons, water percolates into the interior of the earth dissolving silicate deposits in
the process forming a solution rich in silicate salts (Laurs et al. 387). On reaching a cavity
between rocks, the solution settles to create tiny spheres consisting of bright and shiny silicate
salts (Bardosova and Wagner 2). The spheres take time to settle and arrange themselves into an
orderly network. The small size of silica spheres means that a mass of precious opal has multiple
spheres.
Significantly, white light consists of different colors that are only visible through
diffraction. As the white light penetrates the opal gemstone from the top, it passes between
microscopic uniformly arranged spheres and voids leading to the splitting of white light into
different colors. The splits of white light bounce on the surface of the stone generating beautiful
colors of the opal gemstone (Landgrebe et al. 79). Thus, on exposure to light, each sphere

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diffracts the rays causing numerous reflections which lead to the formation of a rainbow-like
spectrum known as the play of color.
Production of Various Colors
The geometric packing of the spheres in the precious opal together with their sizes determines
the nature, quality, and color diffracted. The colors are responsible for the difference in the value
of the different varieties of opal gemstones. Based on the manner in which silicate-rich solution
settles, spheres of different sizes with variation in uniformity form. The occasional earth
movements prevent uniform settlement of the spheres (Laurs et al. 391). Research has identified
that only the spheres "in the order of a tenth of a micron in diameter" can produce the play of
color ("What is Diffraction"). The earth movements before the silicate-solution forms opal
gemstone lead to the random arrangement and different sizes of spheres which leads to the
formation of "potch." Overall, the voids between the spheres and their sizes determine the ability
of the stone to diffract light which leads to the occurrence of play of color. Moving the stone or
source of light makes the rays to fall on the spheres from different angles causing a perceived
change in color.
Different sizes of spheres are responsible for the production of particular colors. For
instance, a small sphere with the diameter less than 150 nanometers produces the violet and blue
colors of the spectrum ("What is Diffraction?"). On the other hand, large spheres of the diameter
of approximately 350 nanometers are responsible for red and orange colors. In essence, "the
more uniform the size of the sphere, the more intense, brilliant, and defined the color will be"
("What is Diffraction?"). For this reason, different types of opal gemstones have a unique
quality.
Diagram of Opal Structure

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Opal gemstone is not crystalline in nature but consists of silicate spheres.
The molecular structure of opal gemstone
Source: “Macroporous Structures, Metal Oxides, Highly Ordered.”
Uses of Opal Gemstone
The uses of opal gemstones vary depending on their nature. Common opal does not exhibit the
play of color. Thus, it is a polishing material. By grinding the stone into a fine powder, it
becomes an excellent abrasive material the polishes metal surfaces (“Natural Gemstones” 7). On
the contrary, precious opal is mainly used in the making of jewelry where it serves various
purposes. It is worth noting that these purposes are based on beliefs and may not apply to all
cultures.
The blue Andean opal is believed to promote transition and inner growth by easing grief.
Green Andean opal connects the mind, emotions, and body facilitating abundance and prosperity
(Gupta 3). Pink Andean opal encourages unconditional love and compassion, heals trauma
caused by emotional difficulties, and promotes inner peace by promoting recovery from grief and
heartbreak.

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Black opal promotes happiness by reducing depression and anxiety. It also facilitates
acceptance and overcoming challenging situations. Blue/green opal encourages openness and
enhances moods (Gupta 4). Water opal is a mood stabilizer while jelly opal instigates survival
instinct that helps in avoiding danger (Gupta 3). Due to these functions, therapists use the stones
to boost moods of people to facilitate openness and quick recovery after a tragic loss.
Conclusion
In summary, opal gemstone is non-crystalline hydrated silicon oxide and is one of the most
valuable gemstones. The three main types of opal are precious opal, fire opal, and common opal.
Common opal is the most widespread type and is less valuable due to its inability to produce
multiple colors. Precious opal is the most valuable stone in this category and is mainly used in
making jewelry and in therapies. Its ability to produces many colors by viewing from different
angles, a trait known as the play of color, is attributed to uniformly aligned and small-sized
spheres which diffract white light to produce a spectrum.

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Works Cited
Bardosova, Maria and Tomas Wagner. Nanomaterials and Nanoarchitectures. Dordrecht:
Springer, 2013.
Gupta, Iyoti. “Color Therapy with Crystals and Gemstones.” International Journal of Research –
GRANTHAALAYAH, 2014, pp. 1-4.
Floter, Annegret et al. The Gemstone Dataset Codebook. International Peace Research Institute,
2005.
Landgrebe, Tom et al. “Relationships between Palaeogeography and Opal Occurrence in
Australia: A Data-Mining Approach.” Computers and Geosciences, vol. 23, 2013, pp. 76-
82.
Laurs, Brendan et al. “Practical Gemmology.’ The Journal of Gemmology, vol. 34, no. 5, 2015,
pp. 379-473.
“Natural Gemstones.” U.S. Department of the Interior: U.S. Geological Survey, 2011, pp. 1-16.
“Macroporous Structures, Metal Oxides, Highly Ordered.” Office for Technology
Commercialization, Technology Marketing Site. University of Minnesota, 2010.
“What is Diffraction?” Causes of Color: Opal, 2018, webexhibits.org/causesofcolor/15F.html.
Accessed 15 Mar. 2018.