Gem of an idea

Natural diamonds are precious, not just for their beauty, but because they are exceedingly scarce.

Formed by a freak of nature as fossilised carbon is forced from the Earth’s mantle into the crust by a supersonic volcanic eruption, diamond mines have to sort through huge amounts of crushed rock to find the stones, and picking out the gems from the dross is a difficult and time-consuming task.

Researchers from the Fraunhofer Institute for Information and Data Processing have joined forces with German machine vision specialist OptoSort to devise a more efficient sorting method.

Combining diamond’s ability to refract light with computer-controlled sorting techniques, they have developed a system which can identify almost all diamonds down to 0.6mm in diameter.

Currently, diamonds are sorted by exploiting one of their lesser-known properties — fluorescence under X-rays. However, this is a slow and rather inefficient method. Worse, it can miss some of the most valuable diamonds, as the more transparent the diamond, the less likely it is to fluoresce — and, of course, pure-white, transparent diamonds are the most highly prized.

The even rarer and more valuable green diamonds hardly fluoresce at all. the only way to find these is by hand-sorting.

The Fraunhofer project, led by Günter Struck, who specialises in visual inspection systems, uses a different property — the ability of diamond to bend light. Diamond has an extremely high refractive index, meaning that it can bend light through a sharp angle. It’s this property which gives the gems their characteristic sparkle.

Struck’s team and OptoSort have developed a system which uses a high-resolution line-scan camera to monitor the stream of diamond-bearing kimberlite rock as it emerges from the crusher.

The camera produces a continuous image, rather than the more usual series of still images from machine vision cameras, and is placed so that it faces the stream of rock as it is thrown from the conveyor belt into an intercepting pit. The falling stones are illuminated from a precisely-calculated angle so that, if they hit a diamond, the light will be refracted into the camera.

The camera is linked to a computer system which controls 200 independent air jets. When a diamond flashes refracted light at the camera, the control system receives a signal telling it exactly where the stone is, and can then actuate an air jet to blow it out of the stream of rock into a receiver.

‘The computer has 60 milliseconds in which to decide whether or not to actuate a jet to blow out a diamond,’ said Struck.

The new equipment needed development work in several areas, including the illumination technique, the rapid image evaluation algorithms, and a constant speed conveyor belt. ‘We need to be certain that the diamond identified will arrive at the appropriate air jet at a particular time,’ said Struck.

According to OptoSort, the system can handle approximately 10 times the material of an X-ray in the same time, and can also detect smaller diamonds. And while it can’t pick out rough black diamonds, it can sort the rarest and most valuable stones.