Opal engagements are becoming the Gem of choice for this generation. Sick and tired of diamonds and wanting something that no one else has? Opal is literally ‘Holding a rainbow in your hand’ – this is the most fitting description of the incredible experience of owning and wearing an opal wedding ring.

Have you ever wondered how all these colours form within Opal rings? White Opal is white because of magnesium oxide, and Black Opal is grey-to-black because of the presence of Iron Oxide.

But what about the rainbows within opal engagement rings? Why do some opal rings display a wide variety of colours while other Opal wedding rings or engagement rings only show blue or green colours? And why are some opal rings display colour which is so much brighter than other opal rings? To answer these questions, let’s start with a short recap of how opals are formed.

How Are Opals Formed?

The history of the formation of Australian opals started thousands of years ago when Australia was covered by a large inland sea (the Great Artesian Basin). The sea covered almost 60 per cent of the land area from Coober Pedy in South Australia up to the Gulf of Carpentaria in the North. When the sea started to dry out, the acidity level in the shallow ground was altered, and massive amounts of organic material (plant and bone) were buried in the silica (sand).

Silica is the first building brick for the creation of opals. The second step of opal formation involves the second major building brick for opals that comes into play: water. As rainwater ran through the ground, it picked up the silica from the sandstone. It carried this silica-rich solution into natural cracks and crevices, forming cavities left by dinosaurs or plant fossils.

Within these cracks and crevices, this solution of hydrated silica gel (silica bonding with water) remained as a deposit. As this deposit hardened, opal and sometimes precious opal were formed. The process of precisely what causes the silica to bond with water and form new molecules is still a scientific mystery!

There is, however, an essential aspect during the process of hydrated silica layering up in the crevices: the precise, perfect way in which the molecules lay themselves in sequential order. Think of the molecules as marbles that are put into a container. You can put them in two ways: you either put them in order, one by one, in perfectly straight alignment. Or you simply throw them into the container, and they land chaotically (unordered) on the inside. These two possibilities graphically explain why there is precious Opal and non-precious Opal (Opal potch).  If the molecules (our marbles) are ordered in perfectly straight lines, Precious Opal will only be found. If the silica molecules form unordered, opal without colour forms. We call this potch.

Why are the rainbow colours in opal engagement rings so bright?

Colours form in many gemstone rings, red in ruby, green in emerald, blue in sapphire, yet none of these rings has the ‘fire’ vibrance and intensity that Australian Opal wedding rings display. To understand this, we have to look at optics: First, we need to understand light and perceived colour.

Light is the visible part of electromagnetic radiation – our eyes perceive radiation waves of different lengths as different colours. Therefore, are colours those specific kinds of waves every object in the world radiates. What is crucial to know is that white light consists of all spectral colours. If you look at the prism in the image below, you can see how white light is diffracted into all the colours of the rainbow.

When white light is being diffracted, it depends on the diffraction angle which colours are eventually to be seen. Red has the longest wavelength and violet the shortest – the remaining colours of the rainbow, orange, yellow, green, blue, and indigo, are to be found in between.

Opals in Opal engagement rings form complex molecular structures that are far more complex than a simple prism, structures that function in a far more complex way. When light hits the spheres of the hydrated silica molecules, it is diffracted, forming complex rainbows. The reason for the different colours lies in the sizes of the molecules and the wavelength of the light. The smaller the molecules are, the smaller is their diffraction angle and therefore, the smaller is the wavelength of the light emitted. The larger the molecules are, the longer is the emitted wavelength of the light. Therefore, do the most significant molecules emit red light and minor violet/blue light. However, as larger silica molecules take longer to form, there are fewer gemstones with larger ones – translated into real-life context: fewer large molecules displaying red colour light, meaningless opal rings showing a red tone. Therefore, these opal rings are more valuable than those rings displaying a blue-ish colour.

Three reasons why Opal Engagement rings are so unusually bright

  1. The contrast of Colour: The effect of contrast of colour explains why the rainbow colours in Black Opal engagement rings are brighter than those in White Opal engagement rings. The key is the body tone of the opal ring. Opal body tones are determined by the presence of iron oxide and magnesium oxide (in white opal).

This effect can be seen in Black Opal rings: the black backing of the body tone allows the rainbow colours to appear to shimmer brighter in the ring. This is the reason why Doublets and Triplet rings are so popular. A black potch’s backing is used to create a contrast that makes the opal ring even brighter.

We call this the thundercloud effect a similar, as seen when a black thundercloud moves behind a rainbow. The black base tone makes the rainbow appear far brighter when we are actually seeing the same rainbow colours!

  1. Lustre also increases the appearance of the colours. Colour lustre is due to surface structure and absorption: At the beginning of this explanation, we learned how common opal and precious opal rings could be distinguished by the molecules’ ordering (or lack of ordering). That specific perfectly aligned order of the molecules also plays an important role in increasing the lustre.

The more perfectly the molecules are aligned (ordered), the brighter the gemstone appears. Why is that? Here, again, the diffraction of light is the key. On a molecular basis, the closer and the more regular molecules are ordered, the more light is diffracted and reflected in the same direction. You can understand this concept if you think of the molecules as tiny mirrors reflecting light. If you have many mirrors positioned in different directions and sizes, they will reflect light in many directions. But if they are positioned in such a way that they reflect the light in the same direction, much more light will be caught by your eyes. An Opal engagement ring is like millions of microscopic mirrors reflecting and refracting the light multiplying the effect.

Absorption also plays a role. Objects (like windows) made of glass are transparent and do therefore not absorb light (the light is not reflected, and glass has almost no lustre). In most opals, that is not the case: their structure allows for light absorption, which is then diffracted and reflected. This is like glass that creates its own internal colour play generating and forming an inner glow!

  1. The last aspect of how our human eyes perceive colours is the Illusion of Luminosity (figure 3). The photograph contains transparency created by superposing several different lengths of gelatine film – to form a series of steps. At the right end of the photograph, the light has passed a single layer of film.

Figure 3: Illusion of Luminosity.

With each step towards the left-hand side, the light has passed one step more than before. Thus, the light had to pass through four layers of film at the left-hand end. Interestingly, we perceive the steps as of continuously decreasing or increasing luminosity. So, the left ends of the small boxes will appear darker than the right-hand side. This illusion, however, vanishes when the boxes are separated from each other by lines (figure 4). The same illusion occurs when we see a colour gradient in an Opal ring – our brain will automatically perceive these colours in the same way as in figure 3.

Figure 4: Vanished Illusion of Luminosity.

To Conclude

Let’s wrap it up! We learned that there a physical and ‘psychological’ reasons for the circumstance that some opal rings appear brighter than others:

  1. The more ordered the molecular structure of the opal rings, the more light they reflect.
  2. The smaller the voids between the molecules, the less milky our opal ring appears.
  3. Colour contrast on dark backgrounds strengthens the colours’ effect.
  4. The illusion of luminosity makes us perceive some colours brighter.
  5. Complementary colours enhance the effects of colour brightness.

Australian opals are simply the world’s most beautiful gemstone ring, with rich, complex colour combinations and several mindboggling unique scientific mysteries working together to give us a gemstone that looks like it has an internal fire. Brilliant, beautiful, and exceptional Opal Engagement rings are lovely and mysterious!

Incredible Planet Staff

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