Dyed Pink Alabaster
Author: Gagan Choudhary
(This article was first published in Gems & Gemology, winter 2009, pg 309-310)
A massive variety of mineral Gypsum (CaSO4.2H2O), alabaster is usually colourless, white or gray, with various shades of yellow, orange, brown or even black due to impurities. Gypsum has a Mohs hardness of 2 and is therefore too soft for use as a gem, but it has been fashioned into beads and carvings. However, a cabochon of alabaster (figure 1) was submitted to the Gem Testing Laboratory of Jaipur, India for identification and it has unexpected orangy pink colour.
Figure 1: This 14.32 ct alabaster cabochon proved to be dyed; note its resemblance to opal or chalcedony.
Also note the whitish chipped areas on the dome due to its very low hardness.
At first glance, the 14.32 ct specimen (17.59 x 12.86 x 8.07 mm) resembled chalcedony or opal because of its translucency. However, the lustre was much too dull and had a distinctive waxy appearance. Fine chips on the dome revealed white powdery white composition (again see, figure 1). The powder was easily removed when wiped with a finger, indicating a very low hardness; this was confirmed by scratching an inconspicuous part of the cabochon with a fingernail. Hence, the specimen could not have been an opal or a chalcedony.
Such low hardness pointed to gypsum, which was confirmed by the spot RI at 1.52 and hydrostatic SG of 2.31. It had a patchy yellow-orange reaction to UV radiation, with a stronger reaction to long wave. Viewed with the desk-model spectroscope, the sample displayed two closely spaced bands in the green region. This absorption pattern is associated with some red dyed quartz, supporting the possibility that the alabaster was dyed. With magnification, the stone clearly exhibited concentrations of red and pink colours in surface reaching fractures (figure 2). Colour variations also were observed in the form of curved bands, with darker zones of concentrated dye in some areas and portions of the original white body colour clearly visible in others (again, see figure 2)
Figure 2: Surface-reaching cracks in the dyed alabaster displayed concentrations of red and pink. See also the curved colour banding on the lower left. Magnified 10x
The sample’s FTIR spectrum displayed complete absorption up to ~5300 cm-1, and bands from 5800 to 5400 cm-1 and 6700 to 6300 cm-1. This pattern resembled that of colourless gypsum in our database, and it is commonly associated with other hydroxyl minerals. The complete absorption of wavelengths in the 3200 – 2700 cm-1 region precluded the detection of any absorption features related to organic dyes or other substances such as wax. Qualitative EDXRF analyses revealed the expected S and Ca, and traces of Sr, but no elements related to a colouring agent were detected.
Dyed gypsum was mentioned in the summer 1963 issue of Gems & Gemology (R. Crowningshield, “Developments and Highlights at the Gem Trade Lab in New York,”p.44) and in the Fall 1964 Gems & Gemology (R.T. Liddicoat, “Developmentsand Highlights at the Gem Trade Lab in Los angeles,”p.219); the latter report stated “A group of pieces of inexpensive antique jewelry contained green and pink coloured beads that proved to be dyed-and-heavily-waxed alabaster.” Although the lustre of the present cabochon also suggested that the wax had been applied in addition to dye, there was no evidence of wax seen when the sample was scratched, and hot point testing was not performed.
All photographs and photomicrographs by Gagan Choudhary