Method for Fingerprinting and Sorting Diamonds

Abstract

Resonant Ultrasound Spectroscopy (RUS) is applied to diamonds (both cut/polished gemstones and rough diamonds) to yield a digital fingerprint from which the stone may be authenticated and sorted according to the structural quality. Diamonds are mined as rough stones from which they undergo examination to determine their value as being gem, or of two different industrial qualities. Fewer than 25% of mined diamonds are worthy of cutting and polishing to yield gems for jewelry. About 40% of the remaining population still have value as industrial diamonds for machine tools, and the rest is ground into dust to provide coatings for grinding applications. Rough stones exist in two conditions being coated and uncoated. The coated stones have a layer of polycrystalline diamond, different from the predominant crystal structure, rendering them opaque. This interferes with optical inspection, as any cracks, or inclusions can't be seen. RUS provides a reliable sorting and fingerprinting system for both cut/polished stones as well as rough diamonds of sufficient structural quality to yield a spectral signature. As high value items, diamonds are shipped around the world, and but sometimes thefts occur. RUS yields a digital fingerprint allowing the identity of an individual stone to be verified upon recovery.

Claims

1. A method of establishing a resonance fingerprint of a cut/polished diamond with resonant ultrasound spectroscopy solely due to its geometrical shape including dimensions comprising the steps of: mounting the stone between piezoelectric mechanical transducers; creating the resonant ultrasound spectrum using a frequency synthesizer as the mechanical driving force to excite a plurality of ultrasonic frequencies over a predetermined frequency range; sensing the resonant mechanical responses with the inverse process using mechanical receiving transducers; connecting a dynamic signal analyzer to receive the response of the stone resulting in a display of the resonance spectrum; and recording that spectrum.

2. The method in claim 1 applied to rough diamonds where the elastic properties are considered, in addition to the shape and dimensions, to yield a fingerprint for stones of sufficient structural rigidity as to yield a repeatable resonance spectrum.

3. The method in claims 1 and 2 where a the spectra of a diamond is recorded, archived, and compared with a new spectra of the same stone to prove, or disprove the original identity.

4. The method in claim 2 to grade rough diamonds into different categories solely due to their structural integrity as observed from the quality (Q factor) of the resonances as determined from resonant ultrasound spectroscopy.

5. The method in claim 2 where the number of high Q resonances are counted, and compared to a single crystal cut diamond to determine if more than one single crystal is present in the bulk structure.

Description

DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 0.50 perfect diamond. Each of the 10 resonances between 1.5 MHz and 3.0 MHz show Q's of 10.sup.4.

[0026] FIG. 2 shows a rough uncoated diamond, simple line shapes, high Q's are evident. Representative of gem quality with more than 1 crystal in the stone. Two traces are shown here corresponding to both the in-phase and quadrature components of the resonances.

[0027] FIG. 3: shows a coated diamond of high quality, containing more than one crystal. These pictures illustrate the quality of the stone from the sharp resonances.

[0028] FIG. 4 shows a rough coated diamond indicating cracks, but with some identifiable resonances.

[0029] FIG. 5 shows a typical rough diamond with no discernable resonances.

[0030] FIG. 6 illustrates the RUS testing of 3 nearly identical 0.50 ct samples to produce different patterns due to differences in geometries. The red, blue and green traces represent three different diamonds.

[0031] FIG. 7 shows a RUS sample mounting stage with diamond between the transducers.

[0032] FIG. 8 shows a schematic diagram of a resonant ultrasound spectrometer. In all resonant ultrasound spectrometers a frequency synthesizer produces an electrical signal that is connected to a piezoelectric crystal, converting the electrical signal to a mechanical vibration. This mechanical transducer contacts a diamond causing it to vibrate. When a natural resonance of the material is found, the vibration occurs throughout the stone. An additional transducer (or 2) in contact with the stone, sense the resulting resonant mechanical displacement, creating an electrical signal which can be amplified, and processed to yield a display of the spectrum. These mechanical vibrations occur solely due to the dimensions, density and elastic properties of the stone. Since the density is nearly identical for all diamonds, whether cut and polished or rough, the resonances are controlled by the geometry and elastic properties.