Optics: measuring and testing – Crystal or gem examination
Patent
1997-05-05
1998-09-01
Pham, Hoa Q.
Optics: measuring and testing
Crystal or gem examination
2504611, 250372, 209578, G01N 2187
Patent
active
058018190
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to distinguishing natural colourless and near-colourless diamond from synthetic colourless or near-colourless diamond. By colourless or near-colourless is meant diamonds which are colour graded from D up to N (inclusive). It is desirable to provide apparatus which is small, compact, safe, and simple to operate.
EP 0 425 426A discloses apparatus for fingerprinting gemstones, particularly diamonds, thereby enabling a specific diamond to be distinguished from other diamonds. The gemstone is excited by light irradiation, which can be ultraviolet light, and the spectrum of the emitted luminescence is detected by measuring the luminescence radiation at a plurality of preselected wavelengths. There is no suggestion of distinguishing natural diamond from synthetic diamond.
EP 0 071 462A describes distinguishing natural from synthetic gemstones, but there is no indication that diamonds are included. The sample gemstone is illuminated with very broad spectrum irradiation, and the total emitted radiation, including internal reflection and fluorescence, is analysed. In the specific example, the lamp used is an ultraviolet lamp with a principal peak at 255 nm, and in general it is indicated that electrons and ultraviolet irradiation can be used, amongst other irradiations.
THE INVENTION
The present invention provides apparatus for and a method of distinguishing natural diamond from synthetic diamond. Preferred and/or optional features of the invention are set forth.
The apparatus of the invention can be provided in a compact and easily portable form suitable for use in jeweller's retail premises, and can be simple and safe in use. Synthetic colourless or near-colourless diamonds can be identified with a high degree of confidence. The invention can also be used to classify as "probably synthetic" a colourless or near-colourless natural diamond that has a surface deposition of low-nitrogen synthetic (CVD) diamond that phosphoresces. Although the invention may not produce a certain classification, the classification which is achievable can confer significant economic benefit. The invention can be used in conjunction with any other method for distinguishing natural diamond from synthetic diamond, to provide a further check. The apparatus can provide a cheap, safe, objective and reproductive means of measuring the decay time of phosphorescence from a diamond.
For the purposes of the present application, phosphorescence denotes any process whereby radiation is emitted by a diamond after the diamond has ceased to be irradiated with ultraviolet light.
The phosphorescence period for diamond may be relatively short, for example milliseconds, or quite long, for example a number of seconds or even minutes. The invention is based upon the observation that there is a strong correlation between synthetic diamonds being colourless or near-colourless and having long phosphorescence decay times. A colourless or near-colourless diamond may be classified as "probably natural" if it has not phosphorescence (the majority of colourless or near-colourless natural diamonds exhibit no phosphorescence) or a short phosphorescence decay time; for example, the phosphorescence time of a natural colourless or near-colourless diamond may be a number of milliseconds or hundredths of a second or less. However, some rare colourless or near colourless natural diamonds of type IIa have phosphorescent times exceeding 1 second. The measured phosphorescence of colourless or near colourless synthetic diamonds consists of a short component (fast decay) and a long component (slow) decay). The fast decay component is typically less than about 1 s. The slow decay component is greater that 1 s and typically remains measurable for 20 s or more and in some cases up to several minutes. The slow decay component gives more useful results. A colourless or near-colourless diamond may be classified as "probably synthetic" and referred for further testing if it has a long phosphorescence time, for example a phosp
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Spear Paul Martyn
Welbourn Christopher
Gersan Establishment
Pham Hoa Q.
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