On September 16 and 17, 2007, members of the DC GIA Alumni Association had the opportunity and privilege to attend two workshops presented by Mr. Alan Hodgkinson of Scotland, a renowned gemologist, author, and instructor. Each day comprised a morning session on “Visual Optics” and an afternoon session on spectroscopy. Slide presentation material and substantial hands-on experience were combined in a manner to enable participants to both understand and practice the subject techniques. The following summary is based on attendance at the presentations given September 17th.
Dr. and Mrs. Alan Hodgkinson
“Visual Optics” is a system created by Mr. Hodgkinson for deriving information on gemological properties by observation of the light patterns seen when a viewer holds a transparent, faceted stone up to his or her eye and looks through the stone toward a concentrated light source. Five principal types of information obtained through this technique were covered: (1) Determination of whether a stone is singly or doubly refractive; (2) estimation of the relative degree of birefringence; (3) estimation of the relative degree of dispersion; (4) recognition of characteristic patterns of particular gems; and (5) estimation and measurement of refractive index.
More specifically, a doubling of the light images (bright spots against a dark background) indicates a doubly refractive stone. The closeness or separation of the two parts of a doubled imaged suggests a lesser or greater degree, respectively, of birefringence. Where the images reveal spectral colors, the amount of elongation or spreading of the spectrum (i.e., stretching of the distance from the red to the violet) increases as dispersion increases. With respect to characteristic patterns, examples included ruby (only red and blue colors are evident), as well as observable differences that can distinguish diamond (tiny, sharp secondary spectral spots), synthetic moissanite (larger, blurred secondary spectral spots), and cubic zirconia (few but primary spectral spots). Concerning refractive index, light images closer to the center of the gem indicate a lower RI, with the distance of the light images from the center of the gem increasing as the RI increases. More precise measurement of RI (and of birefringence and dispersion, where applicable) can also be made through use of a scale and apparatus developed by Mr. Hodgkinson and Mr. William Hanneman know as the Hanneman-Hodgkinson Refractometer.
Alan Hodgkinson playfully poses with two of Bobby Mann’s huge narwhal treasures.
The spectroscope workshop focused on two primary aims: (1) Spectroscope lighting and use techniques; and (2) recognition of characteristic spectrum patterns. Mr. Hodgkinson offered a wealth of tips and tricks for creating an effective viewing geometry and properly lighting different stones. Vivid analogies drawn from air travel, sporting events, and even classic childhood stories helped participants make subtle adjustments to improve the visibility of spectral lines and to remember key distinctions between gemstone species. Critically, a great deal of time during this portion was devoted to practicing with a wide variety of stones, actually trying to disc ern the spectral patterns and to learn by trial, error, and adjustment to become more adept at coaxing secrets and identities from otherwise silent objects of mystery, beauty, and delight. Eureka moments abounded.
The class members at Alan Hodgkinson’s workshop at Bobby Mann’s.
In summary, the chance to learn from a master such as Mr. Hodgkinson was an opportunity not to be missed and for which I, for one, am deeply grateful. I offer my appreciation to Mr. Hodgkinson himself, to Mr. Bobby Mann for his efforts in making these workshops a reality, and to the DC GIA Alumni Association for providing yet another presentation of the highest caliber to feed an insatiable thirst for in-depth knowledge of all things gemological.
Text by Andrea R. Blake
Photos by Bobby Mann