What is more expensive, the tomograph or the precious stone in it?
There are at least three types of pearls: cultured, cultured and natural. Cultured pearls are truly man-made pearls. The mollusk is not involved in its appearance. Only humans take part in the production of artificial pearls from start to finish. Balls made of plastic or glass are coated many times with various pearlescent paints. Imitation pearls cannot contain such a variety of color shades as natural pearls; such pearls are much cheaper. Gemologists can easily identify it. The process of formation of natural and cultured pearls is almost the same. The difference in the case of the formation of natural pearls is that the foreign body is small parasites, but for the formation of cultured pearls the person himself begins the process of maturation of the pearl by inserting the foreign body into the shell of the mollusk. The cost of cultured pearls is much lower, but it is already difficult to determine a fake without looking inside the product. Therefore, the first task that confronted us was to determine the size of the nucleus of cultured pearls using the tomography method.
The research carried out using our microtomograph made it possible to construct a three-dimensional model of the pearl.
But just by seeing the projections of the pearl, we wondered if it was possible to measure the size of the core simply by projections, because visually we see a circle. And indeed the radius was measured “by eye”, and it turned out to be equal to 6.08 mm.
But what will 3D segmentation algorithms tell us? To do this, we implemented the following pipeline for segmentation and measurement of the linear dimensions of a pearl:
Initial approximation for the first active contour
Iterative search for contours with initialization at an adjacent layer
Selecting a range of layers based on visual quality assessment
Selection of 4 parameters of the sphere (radius, center coordinates) using the gradient descent method using the points obtained in step 3
The method we developed allowed us to calculate the parameters of the pearl:
Pearl core radius, mm: 6.12
Total volume of pearl, mm3: 1777
Total core volume, mm3: 962
Core to volume percentage: 54.11%
The segmentation results can be seen in the GIF. Also on the right panel of the gif, for comparison, is the result of the classic Canny edge detector.
The results obtained showed that in the case of simple objects, their sizes can be calculated simply from X-ray images, especially since, for example, pearls can change color and darken when exposed to X-rays for a long time.
But what about more complex asymmetrical stones? For example, we have a sapphire, the cracks in which were restored using heat treatment and filling the cracks with lead glass. How to calculate how much glass there is? Here you can’t do without tomography.
Using Smart Tomo Engine tools, we performed tomographic reconstruction, noise removal and segmentation. The study showed:
Total volume of stone, mm3: 454
Total volume of cracks, mm3: 69
Percentage of cracks to volume: 15.18%
What conclusion can be drawn from our article? It's simple. Microtomography can be used to create a “digital passport” of jewelry. Moreover, there are no longer any questions about reconstruction algorithms (as we have shown, algorithmically it is possible to accurately determine all the features of precious stones). The problem, as often happens, remains only in the legal (legislative) framework.
