How much of a piece of rough is clean and cuttable material?
Or to put it another way:
Specifically, what proportion of the rough stone will end up in the finished faceted gem?
At the earliest stage some weight will have to be removed from the rough to get a piece of rough material that is clean, and able to be dopped, to commence faceting.
Perhaps 10 – 20% of the rough may need to be removed in this initial stage. This may include areas of defects such as inclusions, fractures, twinning, colour-zoning and cracks. Removed material may include that ground away to produce a flat spot on the stone to act as the physical attachment surface.
The rough stone may also have projections that, whilst adding to the total carat weight of the rough, are impossible to be incorporated into any reasonable design, and are thus waste. The piece of rough may also have the opposite of a projection – an indentation. Whilst not technically a crack/fracture as such, it will also affect the final yield, the degree to which will depend on its orientation and depth with respect to the chosen design’s best orientation within the rough’s surfaces.
Does the stone need to be cut to a calibrated size?
If the requirement is to cut the stone to fit a calibrated setting in the jewellery maker’s trade, the yield may be considerably lower – the freedom to cut for maximum yield has been effectively removed. In some situations, if the % loss is going to be significant it may make more sense to have a custom setting fabricated to fit the largest possible stone.
The overall average recovery of commercial (overseas) cutting using flat-faceting techniques is typically below 20%.
That means for commercial overseas cutting you need to begin with a 5+ carat piece of rough to get a 1-carat finished gem back.
The yield from a clean, facetable piece will usually range between 25% and 33% for custom cutting
For an experienced custom cutter in Australia, if you started with a 3+ carat piece of rough, you may get a 1-carat finished gem back.
In essence, the more evenly-shaped the rough, the higher the yield. If the stone is clean, exceptionally well shaped and already tapered towards the culet (i.e. pointy-end!), and you can cut for maximum yield, you might be able to achieve a yield of 40% or more.
So in this ideal scenario, if you started with a 2+ carat piece of rough, you would get a 1-carat finished gem back.