World Of Gems Conference 2017 Proceedings Book

WORLD OF GEMS CONFERENCE V - 52 - SEPTEMBER 2017 C ertain chemical elements are responsible for body color in varying gem materials. Chromium is a chro- mophore responsible for the bright red of ruby and the bright green of emerald (Douma, 2017). The type, number and arrangement of atoms in gemstones can affect what our brain interprets as color. It is very common for the same tran- sition elements in gemstones to produce differing effects in different environments (Gem-A, 2009). RUBY The rich red color of ruby, Al 2 O 3 , is due to an impurity within the aluminum oxide crystal structure. Traces of chromium (Cr 3+ ), replace aluminum (Al 3+ ), about one atom of aluminum per hundred is replaced by a chromium in ruby (Sicree,2007). There is a broad absorption band in the green to yellow that diminishes the transmission of green to yellow light and re- sults in the transmission of red light in ruby spectrum. Ruby has an absorption band around 550nm due to chromium (Fig- ure 1). EMERALD Emerald is a variety of the mineral beryl, Be 3 Al 2 Si 6 O 18 . The chromium ion (Cr 3+ ) replaces aluminum (Al 3+ ) in the mineral’s structure, very similar to that in ruby. Slight differences in the bond strength between ruby and emerald result in shifts in the absorption and transmission bands. There is a slightly weaker absorption from the yellow to the red in emerald. Consequently this sort of absorption diminishes the trans- mission of red light and has a resultant transmission of blue/green light. Emerald’s chromium line is around 600nm (Figure 2). ALEXANDRITE Alexandrite and other gems that exhibit a color change effect depend delicately on the balance of the transmitted light. Daylight contains high proportions of blue and green light; while incandescent light contains a higher balance of red light. Daylight causes the stone to appear green while an in- candescent light source would make the stone appear red (Gem-A, 2009 and Fritsch, and Rossman, 1987). In alexandrite the chromium band is around 580nm intermediate between ruby and emerald (Figure 3). EFFECTS OF CHROMIUM IN RUBY, EMERALD, AND ALEXANDRITE G. Joseph Shihadeh EFFECTS OF CHROMIUM POSTER ABSTRACT FIGURE 1, 2 AND 3: Image courtesy of John Harris gemlab.co.uk