Most gemstones on the market are either cut by machine or cut on rudimentary machines. As a result there are a lot of gemstones out there that have a lot of issues and in my opinion, are less beautiful.
I had mentioned in this post, Considering Gemstone Recuts, about recutting two stones for repair, and one for appearance sake. I would like to go a bit more in depth as to what these cut flaws can look like, and why they are considered flaws.
Please note that I am not a lapidary and do not have as stringent guidelines as they might for what makes a beautiful gem.
Windows
Windowing in gemstones is when the gemstone has what looks like a hole in the middle and the stone is completely see through without sparkle in the middle. What is happening here is that the facets on the pavilion of a stone is cut to such an extreme angle, so that the light goes into the gemstone, and straight back out the bottom. These facet angles on the pavilion can run almost parallel to the table of the gemstone. Here is a good example of a windowed gemstone:
Windowing can occur two different ways, the first being when the gemstone is cut too shallow, typically to preserve a larger face up size for the stone. This cut flaw cannot typically be corrected without significant loss to the faceup size of the gem. This can also affect the value of the gemstone in a big way, losing valuable carat weight and face-up size.
The second occurs when a stone is cut at the wrong angles for the gem variety. The stone typically has a “fat belly” or “fat pavilion” or looks rounded on the bottom, more like a ball than coming to a sharp point, or culet, the way you would expect a gemstone to be cut. These stones are typically cut for weight retention. Here is the stone from my previous post that has this flaw, a silver spinel:
A deep belly/pavilion can typically be corrected with some tweaking by a talented gem cutter.
Tilt Windows
Tilt windows can be seen in every single gemstone variety, including diamond, but are most often seen in stones that have a lower refractive index, such as tourmaline, topaz, quartz and beryl. Tilt windows can be seen when a stone is being viewed from an angle that isn’t straight on. Stones with lower refractive index will show windows more easily. Please note that all of the stones seen in the following examples are cut by talented precision lapidaries.
Tourmaline tilt window:
Slight tilt window seen on the left side of the garnet in this picture:
Slight tilt window in a asscher spinel:
Extinction
Gemstones can be cut to incorrect angles to the point where they black out, or when light goes in, but does not get reflected back to the eye. This type of problem is called extinction, and is a serious flaw because if a stone looks black instead of being a nice color, what’s the point? Some gem cutters expressly cut stones so that facets take turns turning off and on, this effect would be considered being cut for contrast, rather than plain extinction.
The dark blue spinel on the bottom is showing a bit of extinction, while the stones above it (sapphire and spinel, respectively) are not.
Half and Half Shadowing
Half and half is typically another result of less than ideal cutting, and is also a mechanism of physics. It’s a specific type of extinction, so it is a result of light going into a stone and not being reflected back to the viewer’s eye. However, this particular type of extinction is typically seen in an elongated brilliant cut stone such as cushions, radiants and ovals. When these stones are viewed North-South, the phenomenon is very visible, but when the stone is viewed East-West, it can become essentially non-existent.
The dark blue spinel (same from above) on top shows it’s dark side on the upper half.
A purple sapphire shows some half and half darkening on the top half of this stone.
This stone is a spinel, and you can clearly see the half and half as well as a bit of a tilt window.
So, while none of the above are deal breakers when I look at a stone, they are still qualities to take into consideration when purchasing gemstones and looking for beautiful jewelry. Any gemstone variety can have the above cut issues, they are not limited to any one particular gem over another. One way to mostly bypass these issues is by finding a lapidary and checking out their inventory, but that is not a guarantee!
The Gemstone Project 