The study of ancient quarries and mines lies at the interface of geology and archaeology. It is one aspect of the broader discipline of ‘ archaeological geology' or, as it is also known, ‘ geoarchaeology' (Herz and Garrison, 1998; Rapp and Hill, 1998). Simply defined, archaeological geology is the application of geological principles and methods to archaeological objects and sites. In ancient Egypt, as with most other early civilizations, much of what remains consists of stone. There are building stones for temples and pyramids; ornamental stones for vessels, stelae, sarcophagi, statues and other sculptures; and precious stones for jewelry. The archaeological geologist may investigate not only the petrology and uses of these stones, but also the quarries and mines that supplied them, including their layout and operation, extraction and transport technologies, and geologic setting. For Egypt, the study of archaeological stones is well advanced (e.g., Aston et al., 2000; for further information on the quarries and mines see the author's web site at http://www.eeescience.utoledo.edu/egypt/).

As an example of the application of archaeological geology, the present paper looks at emerald mining in ancient Egypt. This review also serves to provide a historical perspective on Canada's own emerald deposit on Regal Ridge in the Pelly Mountain range (near Finlayson Lake) of southeast Yukon Territory. Emeralds were discovered here in 1998 on land owned by Expatriate Resources Ltd. of Vancouver, BC and the property is now being developed by True North Gems Inc., also of Vancouver (Groat et al., 2002). Regal Ridge is the world's youngest emerald discovery, but the world's first emerald mine was in Egypt's Wadi Sikait (Fig. 1,2). The word ‘ wadi' means ‘ valley' in Arabic and ‘ Sikait' is a Bedouin corruption of the ancient name for the site, ‘ Senskete' or ‘ Senskis.'

Egypt was probably the only source of emerald and other green beryl for the ancient civilizations of Europe and the Mediterranean region. Although it has been suggested that the emerald deposit at Habachtal near Salzburg, Austria was worked as early as the Roman period, there is no conclusive evidence that it was known prior to the Middle Ages (Sinkankas, 1981, p. 371-377). However, Giuliani et al. (1998, 2000) have shown that the green beryls from Egypt and Austria are distinguishable by their oxygen-isotopic composition, and such testing, if applied to Roman jewelry, may yet reveal Habachtal's true historical significance.

Egyptian emerald mining occurred not only in Wadi Sikait but also at several other sites within 15 km of this valley, including Gebel Zabara to the northwest, Wadi Nugrus and Wadi Abu Rusheid to the west, Wadi Umm Kabu and Wadi Umm Debaa to the southeast, and Wadi Gimal to the southwest. Mining began first in Wadi Sikait sometime during the Ptolemaic period (late 4^th through mid-1^st centuries BC) with most of the activity occurring in the subsequent Roman (late 1^st century BC through 4^th century AD) and Early Byzantine (5^th through early 6^th centuries AD) periods. All the other mining sites are strictly Roman-Byzantine or Islamic (mid-6^th century AD onward) in date. Beryl mining ceased in Egypt with Spain's discovery of superior-quality Colombian emeralds in the 16^th century AD. It is commonly reported in the literature (e.g., Giuliani et al., 2000, p. 631) that emeralds were used in Egypt as early as the 18^th dynasty (16^th through 14^th centuries BC) of the New Kingdom. This claim, however, is based on the misidentification by archaeologists of amazonite (a green variety of microcline) as emerald (Lucas and Harris, 1962, p. 389-390). The earliest unequivocal evidence for emerald mining in Egypt dates to Ptolemaic times, and even for this period the evidence is scant. It was the Romans who were primarily responsible for developing the mines, and it was they who gave the mining district its ancient name, Mons Smaragdus or ‘ Emerald Mountain'.

Beryl is a beryllium alumino-silicate mineral with the chemical formula Be₃Al₂(Si₆O₁₈). Ordinary beryl is colourless but the presence of various trace impurities gives the gemstone varieties of this mineral their distinctive colours: green emerald, blue to bluish-green aquamarine, pink morganite, red bixbite, and yellow to yellowish-orange heliodor (Sinkankas, 1981, p. 206-235). Beryl almost always occurs as elongated crystals with a hexagonal cross-section. It has a Mohs scratch hardness of 7.5-8, which is exceeded by only a few other gemstones such as chrysoberyl at 8.5, ruby and sapphire corundum at 9, and diamond at 10. Although highly resistant to grinding and cutting, beryl does have a weakly developed basal cleavage. Ancient stonecutters could thus cleave it perpendicular to the crystal axis to produce hexagonal prisms of any desired length.

The name ‘ beryl' comes from the Roman naturalist Pliny the Elder who, writing in the 1^st century AD, used beryllus to refer to a variety of minerals having long, prismatic crystals with hexagonal cross-sections. His smaragdus included the Egyptian beryl among other green stones, but he also recognized its relationship with beryllus: "many people consider the nature of berulli to be similar to, if not identical with, that of smaragdi" (Pliny's Natural History 37.16-20 in Eichholz, 1962,p. 212-227; Healy, 1999, p. 202-203, 241-245). The modern name ‘ emerald' is derived from the ancient smaragdus, a word that can be traced back at least as far as the late 4^th or early 3^rd century BC when it was used by the Greek writer Theophrastus as a catchall for green gemstones (Theophrastus' On Stones 23-27 in Caley and Richards, 1956, p. 50-51, 97-109). The Egyptian green beryl, however, was almost certainly unknown to him. The first mention of beryl (smaragdos) mining in Egypt was by the Greek geographer Strabo about 24 BC (Strabo's Geography 17.1.45 in Jones, 1959, p. 120-121). When mining began in Wadi Sikait is not known precisely but, given the almost total absence of green beryl in Ptolemaic jewelry, it must have been late in the Ptolemaic period and probably not much before Strabo wrote about it.

True emerald has a bright, uniform, medium to dark green colour and is transparent. Beryls of this quality are very rare not only in Wadi Sikait but throughout the surrounding beryl-mining region. The Egyptian green beryl almost always has a pale colour and a cloudy translucency (due to abundant fluid inclusions), and it also commonly contains minute mineral inclusions (usually phlogopite or actinolite, or their weathered clay-mineral equivalents). It is ironic, therefore, that Egypt's famous ‘ emerald mines' produced very few true emeralds.

The poor quality of Egyptian green beryl has been attested to repeatedly in both the ancient and modern literature (for a partial summary see Sinkankas, 1981, p. 542-548). For example, Pliny the Elder complained that the "Ethiopian [i.e., Egyptian] smaragdus is … rarely flawless or uniform in tint" (Pliny's Natural History37.18.69 in Eichholz, 1962, p. 218-219). Although Egypt's ordinary green beryl may not have been highly esteemed by the Romans, it was still clearly much valued by them for jewelry. It was the hardest green gemstone available to them and it also had the added mystic of coming from fabled Egypt. Perhaps another appeal of beryl was its naturally faceted hexagonal prisms that mimicked the more costly cut gemstones. Unlike in recent centuries, when beryl has been ground into faceted stones, the Romans used the natural hexagonal prisms cleaved from crystals. These were either fixed into metal settings or drilled along the prism axis and strung as beads (Fig. 3).

The geologic occurrence of beryl in Wadi Sikait has been well described by Basta and Zaki (1961) and Abdalla and Mohamed (1999), and their findings are consistent with what is known generally about the origins of beryl deposits elsewhere (Sinkankas, 1981, p. 339-356). For green beryl to form, two relatively rare elements need to be present: beryllium and chromium. Beryllium-bearing minerals are most commonly associated with hydrothermal veins that are offshoots of silicic magma bodies. At Wadi Sikait, such a magma body produced the granite with its quartz and pegmatite veins. These veins generally vary from a few centimetresup to one metre in thickness, and have intruded all older rock units in the area. The veins are now much deformed and so commonly appear as discontinuous lenticular bands and pods. The occurrence of beryl is intimately linked with these Be-enriched veins, especially those of milky quartz, which are the most plentiful (Fig. 4).

Only minute amounts of Cr substituting for Al in the beryl crystal structure are needed to give the mineral a green colour, with darker shades produced by higher Cr concentrations. Although V can also colour beryl green, chemical analyses have shown that Cr is the colorant for Wadi Sikait beryl. Unpublished analyses of five beryl specimens from Wadi Sikait reveal Cr concentrations ranging from 150 to 1013 ppm, and averaging 552 ppm (EGSMA, 1992, Table 3.4; A. El Dougdoug, Geology Dept., Cairo University, Egypt, pers. comm.). Chromium is commonly found in rocks of mafic composition, where it occursas an impurity in mica and amphibole minerals through substitution for the Al and Fe in their crystal structures. In Wadi Sikait, these rocks are represented by phlogopite and actinolite schists in the schist melange. The actinolite schist occurs as thin sheets and lenses within the much more abundant phlogopite schist.

Beryl occurs mainly in the phlogopite schist and quartz/pegmatite veins, and is restricted to within tens of centimetres of their contact. It is found as individual crystals and, more often, as small clusters of crystals. Crystals can be up to 3 cm in length but most are much shorter. The beryl in the quartz/ pegmatite veins varies from colourless or white to light green but in the phlogopite schist the colour ranges from light to dark green. Given that the schist is a source of Cr, it is not surprising that this rock would have the greener beryl, including true emerald. The geologic occurrence of green beryl at Wadi Sikait is very similar to that at Canada's Regal Ridge. Here the green beryl, which is coloured by Cr, occurs within mica schist close to its contact with quartz veins spawned by a nearby granite intrusion (Groat et al., 2002, p. 1330). Contrary to what Giuliani et al. (1998, p. 513-514) claim, Wadi Sikait is not a Type II emerald deposit, where Be-bearing hydrothermal fluids permeated Cr- or V-bearing rocks along thrust faults or shear zones. Both Wadi Sikait and Regal Ridge classify as Type I emerald deposits, where Be-bearing quartz or pegmatite veins from a granitic pluton intruded Cr- or V-bearing mafic or ultramafic rocks.

In Egypt's Wadi Sikait, ancient miners extracted emerald and other green beryl from the contact zone between phlogopite schist and intrusive quartz and pegmatite veins. This is essentially the same geologic occurrence as the beryl on Canada's Regal Ridge. The world's oldest emerald mine (Wadi Sikait) and youngest emerald discovery (Regal Ridge) are separated by two millennia, yet are linked by not only their similar geology but also their common purpose – to satisfy people's desire for emerald jewelry. It is through the practice of archaeological geology that such links are made.