In the world of coloured gemstones, sapphires are at the helm. They govern the kingdom of gemstones with an aluminium oxide fist. It is the second hardest in the Mohs’ scale of hardness, just under diamonds. But sapphire colour isn’t always mixed within the stone itself, in fact it is due to impurities. We describe this as being “allochromatic”. The sapphire today that I looked at through the microscope was a sapphire which showed this concept quite vividly!
Colour Zoning Characteristics
In this microscopic image we see that there are a whole bunch of vertical lines which run straight down the stone. A couple of these vertical lines are in-fact facet edges, while the rest is part of the colour zoning itself. The straight line running straight down the middle of the image is a facet edge. How do we know this? well in the image this line is much more focussed and precise than the other lines running vertically. Other facet edges include the line at the very left of the stone, which once again is more defined than the other lines. Next, we have all of the other lines in-between, showing different variations of the blue colour. From cornflower to royal these bars come together to create a brighter blue.
The next question to be asked is: how do we know that this is the colour zoning? They are much closer together, compared to the facet lines. The colour zoning has a faded appearance, which creates this said “zone”. This means that when looking at the stone to the naked eye, we should be able to see said zone, where the intensity/saturation of colour is much in that area than any other parts of the stone.
Accompanying the colour zoning, there are a couple crystal inclusions which to the naked eye cannot be seen. But enough of that let’s have the look of the same stone in a different magnification.
A Different Magnification
In this image we have the same stone in the same orientation but relatively more zoomed out than the previous image. Observing the colour zoning we can see that the zoning is through the whole gemstone, meaning that in the gemstone market this stone would be able to hold its own being a “blue” sapphire. At first glance, to the inexperienced eye, the colour zoning looks like someone hasn’t cleaned the stone properly. However to the experienced eye, we can see the colour zoning running rampant throughout the stone.
The crystal inclusion which we had described before is still visible under this magnification but is starting to look more and more minuscule, making this gemstone an eye clean stone.
The next time you pick up a colour zoned sapphire, hopefully you can tell that it has colour zoning! I’ll see you again next week!