Identification of naturally blue diamonds is the easiest to carry out and is unequivocal. Structurally natural blue diamonds fall under Type II b and have the property of electrical, conductivity. Artificially colored blue diamonds come in most cases from the slightly yellowish-colored hues and can be classified under Type I a. They have no conductivity. This difference can be easily demonstrated if the diamond to be tested is clamped between two metal electrodes and a circuit current is applied. The current conduction can be read from a connected measuring instrument. A positive reading indicates that this is an electrically conductive and hence a natural blue diamond.
Stones irradiated in the cyclotron can also be recognized fairly easily. In the microscope the so-called "umbrella effect" can be observed through the table, provided that the brilliant was irradiated from the pavilion part upwards, which is the case with 98% of all stones. In the rarer cases where radiation extends to the crown, a black ring within the girdle can be seen. In any event, cyclone-treated diamonds are rarely encountered in gemological examination practice.
Distinguishing between different natural and artificially produced colors is not always as easy or so unequivocal. In case of doubt the examination should be left in every case to a professional gem testing laboratory. Distinguishing features will be explained here only with the aid of the spectroscope, the use of Which, however, presupposes considerable practical experience and is not successful in all cases.
Natural yellow diamonds show characteristic absorption lines at 414.5 (clear), 452, 465, 478.5 (weak), 435, 423, 401, and 390 nm (seldom discernible).
Natural brownish or brown diamonds show a single clear
line at 504 nm
accompanied by a weaker double line at 498 and 537 nm.
Diamonds which are artificially colored yellow to yellow-brown show a clear absorption line at 595 nm which never occurs in naturally colored diamonds. This strong line is accompanied mostly by a weaker pair of lines at 498 and 504 nm. In addition to these characteristic absorption lines, the lines of the natural spectrum of the yellow series occur, as, in general, diamonds from that series which are tinted light-yellowish or light-brownish serve as the initial product for the purpose of making improvements in color.
Diamonds which are artificially colored brownish to brownish-red show in addition to the diagnostic recognition line of 594 nm weaker lines at 637,620 and 610 nm, which however can often be recognized only with difficulty. The spectrum of diamonds from the yellow series can also to be seen here.
Diamonds which are artificially colored deep brown show two lines of equal strength at 498 and 504 nm which - if both are equally clear - are a definite sign of artificial coloring. The spectrum of diamonds from the yellow series can also be observed.
Diamonds which are artificially colored bright- to dark-bluish-green also show a double line of equal strength at 498 and 504 nm, and are noticeable because of an unnatural and often "greasy" color impression.
Diamonds which are artificially colored pale pink have hitherto been considered to be chance products. However irradiated pale pink diamond are appearing with increasing frequency in the trade, and from this it can be concluded that in the meantime these colors too can be intentionally produced by artificial means. The distinguishing features known to date are: apricot to salmon-colored fluorescence in long-wave UV light and an absorption line at 637 nm.
Special care is called for in each case, and a doubtful stone should be sent to an appropriate institute for more accurate examination (e.g. for spectrophotometric analysis). Particularly because of the extraordinarily high prices which are being paid for naturally colored pale-pink diamonds, extreme care and caution should be exercised in judging them.
| Irradiated diamonds | Summary table |