Virtual rough planning for gemstone faceting

Rough planning is an important operation to maximize the weight of the gemstone to be cut from a rough piece. This operation is routinely done for diamonds, with the use of high precision scanners and automated systems to locate the best shape, size and orientation of the gem for better yield. However, for colored stones, the planning is usually done by eye, according to the experience of the cutter. That can lead to costly errors, as the best shape and location of the future gem can be quite difficult to imagine in some cases, and even a fraction of millimeter of miss-orientation can significantly lower the weight of the future gem. 

Fortunately, the advance of computer technologies can help us with this operation, with no need of expensive tools and software. Here I explain step by step the process of virtual rough planning for a piece of exceptionally rare green sphalerite from Bulgaria. Clean and pure green sphalerites of this size (70.11 cts) are very scarce and this rough well worth some additional time of work for the best planning possible!

To avoid the need of manual preforming of the rough, when every touch of the grinding disc cannot be un-done, I proceed to 3D-scan the piece of rough to study it on the computer screen and find the best options of shapes and orientations for the future gemstones. Here is a video I prepared for the demonstration of this process:

Some additional details of this process are explained below.

1. 3D scanning. This is done by means of photogrammetry, the 3D model is constructed on the base of photographs, taken from different angles to the object. You only need your camera (Nikon DSLR here) and a special software (3DF Zephyr Free – yes, free version works great!). I placed the camera on desktop tripod and the stone attached with blue tac to a metal rod in the center of a rotating table. I took 24 shots in total, rotating the table manually (no high precision in rotation angles needed), 16 from the same height of the stone (lens perpendicular to the rotation axis) plus two additional tires of 4 shots elevating the camera several centimeters twice to cover better the top view. You can add more shots or even add some shots from below, lowering the camera, to cover the bottom view, but it was not necessary in this case.

Below is the picture of the reconstruction of camera locations made by Zephyr. In fact, I was rotating the stone, not the camera around the stone, but the result is the same. They extract the focal lens distance from the image info and make all the reconstructions based on the images. It’s a kind of magic! Even the size of the stone was reproduced correctly to a millimeter scale without any input from me about the size of the object!

Important point: transparent and brilliant objects will confuse Zephyr, so we need to paint the stone with some matte paint and place some distinct marks on its surface. These marks will be helpful for a better 3D scanning and also on the final stage, when we need to bring the results of the virtual planning to the real stone. Here are some of the shots used for 3D reconstruction:

2. Once the model is scanned, we export it from Zephyr as OBJ file. Good thing it will also save the surface texture of the object, so our marks will be there! Now we can import the rough model to the 3D software of your choice (Blender, Rhinoceros, AutoCAD, Matrix, etc) and start planning!

For that, I used Blender 2.81. It is free software, VERY powerful, but it requires some training to be used. You can cut off the support base of your 3D rough model and make some additional adjustments, like decimate the mesh to delete unnecessary details, etc.

3. Now we need to import the faceting designs that we want to try to the same 3D software where we have our rough model. For that, we must first export them from GemCAD or Gem Cut Studio as DXF, STL or OBJ files.

4. Finally, the game begins. We move, scale and rotate our faceting designs to manually find the best fitting position and size of each design inside the rough model. Once we get it, we can easily check the volume of the virtual stone in the Properties box of the software, to compare which design will have larger volume and hence the weight.

Alternatively, we can also do a “virtual preforming” – to “saw” the rough in two or more parts, to cut off some areas, according to the existing fissures or just to go little by little, like using grinding disk, but with the possibility to go back at any moment!

It would be great to have some day a software capable of finding automatically the best location and size of a faceting design inside a given 3D rough model, like the software used for diamond planning. So far, I have not found such options, so I have to do it manually.

5. Once the best design is chosen, we need to transfer the location of the central axis of the future gem and its girdle plane to the surface of the real stone. For that, we add those elements to the virtual stone, to find the points of intersection with the rough model, and then find the corresponding points on the real stone, taking references of the marks on its surface.

One useful tip here is to scratch gently the surface of the rough in some points before the painting is removed, and later to highlight those scratches with a sharpie. 

Now the stone is ready for accurate doping and further faceting!

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