Lets Talk Cuts

The 4 Cs are everything when it comes to gemstones (carat weight, cut, colour and clarity), the one part of the 4 Cs that can have the greatest effect is cut. The cut dictates the how the other 3 turn out. Theres 2 types of cuts precision cuts and commercial cuts, the difference between the 2 is precision cuts try to make all 4 Cs as good as possible whereas a commercial cut only tries to have the maximum carat weight without taking into count the colour or clarity of the stone. The below photos show the difference, by not having the correct angles for refraction you can see theres large windows. A window is a area in the stone where the light passes through the stone instead of being reflected back through the top of the stone.






The following photos show the different parts of a cut stone as well as the most common cuts that are available. Note on a commercial cut the girdle will not always be straight






Theres also rare cuts that arent often used. The following photos show a design of a cut for interest and the pharaoh eye cut is one I really like





As you can see the cut is one of the most important aspect of a gemstone. Its a gift to be able to pick a cut that maximises the clarity and colour without losing carat weight.

Feel free to add more information I havent broached, I became distracted and lost my train of thought so not sure if I have covered everything I wanted to when I started this thread.

Thanks for giving the subject a bit of a nudge.(I was gunna say clarity) :gemstone:

Yes indeed :Y:

On the subject of windows, a properly-cut stone can still display a "tilt window" - that is, it will show a window when the stone is tilted far enough away from the normal face-up viewing angle as the tilt away from normal breaches the stones critical angle from the perspective of the viewer. Of course, a stone is intended to be viewed straight down on the crown mostly so this isn't considered a problem It's more noticable in lower RI stones like feldspar and quartz since their higher critical angle (41.5 degrees and 40.5 degrees respectively off the top of my head anyway) typically gives less room between the culet angle and critical. It tends to be less noticable with really high RI stones like zircons, with a critical angle of around 30.5 degrees.

I can see rather a lot of hours in cutting that second-last diagram

Let's not forget the other one, our friend extinction. These are the black areas in the stone where no light returns. Like in this ruby below.



This happens because the angle of the cut on the bottom of the stone, the pavilion, is to steep.
Light enters through the top of the stone, the table, strikes the first pavilion facet and is reflected internally as desired but then hits the second facet at to steep an angle which then 'leaks' out the stone. No or little light is returned through the table to the eye so a black or darkened area is visible.

Although in both windowing and extinction, light has leaked out the stone with windowing a lot of light also enters the stone from the bottom through the pavilion to 'reach' the eye. With extinction the eye receives little or no light so the area is darkened.



Generally neither are desirable as it takes away from the brilliance of the stone but both windowing and extinction can be used to good effect. Although generally the extinction is used to create patterns or contrast in the stone..

Another form of extinction is caused by the stone containing Iron. Iron absorbs light. That is why many stones that contain iron are generally dark. Stones such as Australian sapphires, Thai rubies (Metamorphic including Burmese have a low iron content), Tourmaline containing iron is generally dark or black. Almandine garnets have a high iron. Both Hessonite and spessonite are orange garnets. The Hessonite is coloured by Iron the Spessonite by Mangenese. A Spessonite will always transmit light better than a Hessonite of similar characteristics.
Although aquamarine gets it's colour from Iron impureties within colourless Beryl.

Bottom line is a stone has to be cut taking things like composition of the stone. With a lot of stones containing a high iron content you want the light to return in a short a distance as possible. Generally.
The ruby above is a Thai ruby that suffers from both a bad cut and a high iron content. A Burmese (or any other metamorphic ruby) ruby would partially mask the bad cut because of the ability to covert UV to visible light, giving the stone an inner glow.



A Thai and Burmese ruby side by side.

Cool thread.

Some styles of cuts are effected by windowing, extinction and head shadow more so than others. Long thin crystals are very difficult to work with as they often require very long facets to make best use of the material and maintian weight. A very common cut, both commercial and some precision variations, is the baguette style.



This design can be stretched as far as you like without having to obey the ratios. However, this cut is very prone to windowing and head shadow due to the nature of long, stepped facets. Also its very boring.

With a bit of imagination its possible to alter the baguette concept to maximise scintilation and minimise both head shadow and windowing. This is my new Ice Pick design. It does not solve the typical issues encountered when cutting very long stones but it uses strategic windowing and head shadow to create more effect in the middle section of the stone which is normally dull and boring.



The ultimate fix for windowing and headshadow in this style is to go the next step and throw a few concaved facets in the middle section of the stone.

Great topic :clap: Another element for producing windowing is the ratio of length to width, and is particularly apparent in oval cuts, anything over around 1.25-1.3 will be prone to windowing as the optimum angles can't be maintained.

Lefty said:

I can see rather a lot of hours in cutting that second-last diagram

Click to expand...

Yeah the outcome looks nice so with the right rough would be worth it

Lefty said:

I can see rather a lot of hours in cutting that second-last diagram

Click to expand...

I couldn't cut the 2nd last one without angles and indexes.

The amount of highly talented craftsmen on the forum is amazing-all forms.

On the subject of the 4 Cs, colour is of interest to me me as I am considering how to have a large dark sapphire cut. I understand that the table is blue with green cross table.
How would colour be maximised in a stone like this and what would be the problem with choosing green as the table as it appears to allow more light through than the blue.
(Images 1&2 natural light, 3&4 LED)
Thanks,
Martyz

WOW thats HUGE

Choose the colour you want for your table and cut for brilliance. I would not cut for a bicolour or parti table unless the blue table is minimal and the parti is showing a decent colour mix, but if that is the case, is there enough depth of stone to slice in two through the cross table and then cut a diagram such as Blue Ice and having a blue table? Would make nice drop earrings!

I would be tempted to cut for the blue as it is a great colour. Because either way you will probably have a dark stone. Best way to check for final result/colour is to conduct a white paper test. https://www.gemsociety.org/article/using-white-paper-test/

Also a dichroscope if available will help indicate true colour.
https://www.gemsociety.org/article/just-ask-jeff-what-is-a-dichroscope-and-how-do-i-use-one/

I'd be tempted for a shallow as possible cut with a low facet count on the pavillion to act as large (relative) mirrors to bounce back 'winks' of light.
What ever you decide, it would be great to see the finished result.

Thanks for your advice Pat and Mr Magoo.
As usual this forum proves to be great source of experienced and knowledgeable people who are happy to give advice.
I was almost chased out of a certain sapphire shop outside of Mackay because I had few questions for the gem cutter. Apparently there is no money in answering peoples questions and as Im not looking to make any money from this stone the sales assistant couldnt help.
Anyway, its good to have passionate people out there to help navigate the technicalities of this difficult rock.
Im getting there.
Martyz