Investigations on ancient beads from the Sultanate of Oman (Ra's al-Hadd - Southern Oman)

Abstract

A study has been carried out on the mineralogical composition of stone beads found in the course of archaeological excavations carried out at the site of Ra's al-Hadd in Southern Oman. The data provide information about the raw material and technology used for beads production. Several beads and micro-beads made of white paste of different shapes and sizes have been analysed. They range, in size, from two to four millimetres in diameter, two to fifteen millimetres in height, and the diameter of the hole is approximately one millimetre. The shape is tubular for the biggest bead, cylindrical for slender beads and ring-shaped for micro-beads. The beads were analysed using X-ray diffraction (XRD); the method was optimised to minimise the amount of the sample necessary for analysis (15 milligrams were necessary using special sample holders while in traditional holders 300 milligrams are needed). XRD analysis revealed that the beads arc made of “synthetic enstatite”, a magnesium silicate belonging to pyroxene group (orthopy-roxene Mg2Si2O6). The relative intensity of the main peaks confirms that the material is “synthetic enstatite”. Natural enstatite is a hard mineral (its hardness is five or six on the Mohs scale), therefore we propose that a softer magnesium-bearing mineral was used: steatite (Mg5Si4O10 (OH)2 commonly known as massive talc - hardness one on the Mohs scale). The beads might have been worked from soft steatite bulk material and then hardened by transformation of the steatite to synthetic enstatite during firing at about 1000 °C so that the formerly soft beads became hard and durable. Some preliminary heating experiments on talc bulk fired at about 1000 °C was carried out to compare powder diffractograms of the beads and the fired massive talc; a good overall resemblance of the two patterns is evident. The raw material of the examined beads might have came from the ultramafic rocks of Oman and it is very likely that these beads were produced from locally available raw materials.