Corundum Mineral - Sapphire Chemistry

_{A blue sapphire with a visible crystal structure}

For the first-time reader, the term corundum might sound awfully similar to the term conundrum. But it is anything but. Those who have been engaged in any way with developments in the gemstone world, or maybe even have an interest in mineralogy, would know that the term corundum is associated with sapphires.

However, sapphires are not the only type of corundum crystal or mineral in the market. There is another key mineral that is worth considering. This and other key factors that determine the character corundum minerals, with the notable variant of the sapphire will be the subject of this Education page.

The Sources of Corundum Mineral

The term corundum refers to the hardened mineral that is a constituent of rock formations of various types: Ranging from sedimentary (hardened organic deposits), metamorphic (rock formations that have transformed owing to geological effects), and igneous (hardened lava and magma). The presence of corundum crystals in these various rock formations is indicative of their ancient origins.

As corundum crystals like sapphires form deep within the Earth’s surface over the course of millions of years and in the process they become part of these other rock and mineral formations. This is why corundum crystals like sapphires that are found in nature often vary in terms of their sources. With primary (rock formations) and secondary (alluvial deposits) leading the way.

This is why sapphires are not sourced from all parts of the world. Whilst newer corundum deposits are being found, only a handful of nations lead the way in the corundum souring, and even fewer in terms of providing gem-quality corundum crystals.

What Elements Make Up the Corundum?

When it comes to the main elements that corundum the key constituent is aluminium oxide. Aluminium oxide is a naturally transparent mineral, that shares certain similarities with another crystal formation known as Hematite. However the latter is dominant in iron atoms, whereas the corundum is dominant in aluminium.

Corundum is known for its extremely hardened character. Among the hardest natural minerals in the world (scoring a 9.0 on the Mohs scale for hardness). Corundum known by its chemical dentition Al2O3 derives its durability from the shortness and firmness that holds the oxygen and aluminium atoms together. However, the variations in colour are determined by other atoms that are found in a minute but adequate amounts to change their visual character. This takes us to the next point on the types of corundum crystals.

Different Types of Corundum

When one speaks of the different types of corundum what we are getting at is its visual character. At its core, corundum crystals are basically the same with the key difference being their external hue. People might not know this, but corundum crystals, are actually colourless in their pure state. The colour that is visible to the observer, that we have come to identify with them is a function of external elements.

The most notable corundum types are sapphires and rubies. This might surprise some readers, as both sapphires and rubies have the same mineral character but differ in the composition of trace minerals that they contain. Sapphires and rubies derive their colour from the presence of trace minerals like iron, titanium, and chromium. 

The trace minerals which are found in tiny amounts alter the way the stone interacts with light, thus its colour. In sapphires, the famous blue is a result of iron and titanium, with the presence of chromium pushing it closer to pink or magenta. Corundum crystals with no trace minerals are known as white sapphires, as they feature no distinct tones, but come with a light warm tone.

What Makes Sapphires Different

In the gemstone world, the two key factors of value are the beauty of the stone and its rarity. Whilst the durability of the stone measured by its hardness is also an important factor. With corundum minerals their hardness is a well-established quality, however, the main driver of value in these stones is the difficulty of coming by good quality gems in nature. This is due to a number of factors.

The first is the formation of these gems in nature. As these gems formed over the course of long periods in history, deep within the Earth's surface, often originating in the crust. However, their formation requires the presence of certain key minerals and also the absence of others.

The notable feature in the development of corundum minerals is the absence of silicon. For the corundum to form an environment free of silicon deposits is essential. This is a bit of a problem because silicon is a very common element in nature, hence impeding their abundance. This is why sapphire and other corundum crystals like rubies are very rare.

Optical Character of Corundum

As noted the value of corundum lies in its visual character. Whilst colour is naturally the dominant factor, with blue being the most recognised and valued, other factors are equal. But all of this rests on how the corundum mineral interacts with light. The visibility of the stone and its distinct hues and glow rests on how the materials interact with light: a function of how photons interact with atoms of the gemstone and the type of effects they produce.

This brings us to the subject of diaphaneity: A technical term which refers to the ability of the material to transmit light. Which breaks down into the transparency, translucency and opacity of the material. In this regard, corundum minerals are an interesting case in that they feature all three levels. In the stone-specific corundum mineral, depending on the colour and vibrancy of the stone, or the lack thereof can be either transparent (e.g. a cut and polished white sapphire); translucent (a light pink sapphire with some visible light passing through it), to a deep blue sapphire that allows no light to pass through.

In this regard corundum stones, depending on their trace mineral character and colour presence, provide a wide range of choices for customers to choose from. That is if your interest is in gem-quality corundum stones that can be used in fine jewellery like sapphire engagement rings. This takes us to the next point.

The Value of Corundum: Sapphire Gemstones

In simple terms, the value of corundum, notably sapphires is due to their rarity in nature. However, for something to be valuable, it is not enough that it be rare, but that it also contains distinct features which make it attractive to the masses. Points which we have already engaged. So when it comes to the value of corundum stones, specifically the sapphire variant, what we are looking for is the quality that makes it gem worthy stone.

Such as the stone’s rich colour and even the type of colour. As noted, sapphires despite being rare in nature are not impossible to come by. Sapphires today are sourced from the number of different countries and their value is determined by the individual quality of each gemstone. This is determined by the colour: the richness of the hue, the type of tone, and the spread.

Notable here is the phenomenon of colour zoning: where the colour in a sapphire is not uniformly distributed. Which affects the quality of the stone. So sapphires with less colour zoning are more valuable.

Then there are certain types of sapphires: such as the Royal blue, or Cornflower blue sapphires which are valued for their distinct tones. Followed by rarer varieties of sapphire colours like the Paradisaical sapphire, which demand a high markup. Ultimately the value of the corundum crystal is determined by a combination of factors: most notably its carat, cut and clarity.