Traditionally, once they are shaped, but before they are mounted, diamonds are rated by the “4 Cs” — clarity, color, carat, and cut. Modern gemology, however, grades finished diamonds by a more accurate standard: light performance. The quality of how a diamond innately captures and “plays” with light separates a superior gemstone from a flawed one. Supporting 10 million people and recording a profit of $1,010 billion in 2006, but facing a global supply crunch and competition from gem-quality synthetic diamonds, the diamond industry must get superior stones on the market quickly and reliably. For quality assurance, many firms have turned to the BrillianceScope™ Analyzer from GemEx (Mequon, WI) to measure the light performance of finished diamonds in a comparative test against some of the industry’s highest-rated stones. GemEx utilized the RGS™ Rapid Guide Screws from Kerk Motion Products (Hollis, NH) to maintain the precision of the BrillianceScope’s testing mechanism.
Primarily used by gemcutters, the BrillianceScope, about the size of a desktop computer, is an imaging spectrophotometer using a band pass filter that works in the visible light range of 400 to 700 nm. The precision of the test depends on the consistent movement of the ring light as the test is performed. Due to the comparative scale, the diamonds being tested must be in the same position each time an analysis is run, meaning the rail must stop at the same points with each test. The same scale demands that each diamond receives the exact same test according to its shape (Round Brilliants are compared only to Round Brilliants, Hearts only to Hearts, etc.). After in-house GemEx designs failed to produce necessary test precision, Kerk provided the RGS, a stainless steel lead screw combined with an aluminum guide and carriage. The RGS provided a stable base for the ring light and did not sway or vibrate in midtest (movement of the light beam would throw the test results). The screw is integral in each of the five movements that the ring light makes, with a repeatability factor of ±5%.
The device measures a finished diamond in a controlled lighting environment consisting of multiple lighting angles. To initiate the evaluation process, a diamond is centered on its crown (upside-down) on a sheet of optical glass within a three-inch integrated sphere at the top of the BrillianceScope, which is then shut to minimize external light. At the bottom of the sphere is a 1" hole, which creates the cone of light that enters the diamond. The 2.5" ring light lies just beyond the hole, and passes under it during the course of a test. As the light approaches, passes under, and recedes from the hole (traveling along the length of the screw), the five different angles of light are created that the BrillianceScope uses for its test. The movement is done via a motion system consisting of a stepper motor attached to a rail (and the RGS), upon which is situated the ring light. A black-and-white camera observes the tests (as a spectrophotometer, the BrillianceScope calculates color based on spectral response). These images are viewed on a computer, and at each angle of the test, a picture is taken. The five pictures are analyzed, pixel by pixel, for the amount of flashes and other light responses emanating from the diamond.
Light performance is rated by three criteria: white light, colored light, and scintillation. Diamonds being tested are measured by past tests; during the development of the BrillianceScope, GemEx measured thousands of diamonds and determined which rated best and worst for light performance. The upper and lower ends of the testing scale were set from these initial tests. White light, referred as “brightness” or “brilliance,” is the combination of all white light reflecting from the surface and interior of a finished diamond. Colored light, or “fire,” grades the flares of colored light emitted from a diamond. Scintillation, also termed “sparkle,” describes the flashes of light produced when the diamond, light source, or observer moves.
The device only tests diamonds; colored stones are judged by different standards. Currently, 160 BrillianceScopes are in use by customers. GemEx retains ownership of the machines, while cutting facilities perform the actual analyses. Once the test is complete, a report is generated that rates the diamond on a graph; it is this report that is then sold to retailers. Light performance, split into the brilliance, fire, and sparkle categories, is listed in a range from “low” to “very high.” If, after the BrillianceScope test, the quality is found to be not up to a particular standard (set by the retailer), the diamond is sent back to the cutter for reshaping or further evaluation — even “ideal cut” stones will test poorly if the structure of the diamond itself is flawed. The report also includes 4C information that is provided independent of GemEx and the BrillianceScope.
For more information, contact Jim Bostwick, Applications/Sales Engineer, Kerk Motion Products, at