A main area of concern is that of identification of gem rough in the field. Many,if not most coloured gemstones we see today in jewellery stores are coated,fracturefilled, dyed and/or irradiated in linear accelerators in efforts to enhance their appearance. Heat treatment is the most common technique used. The gemstones that one sees in the jewellery store,the vivid violet-blue tanzanites,midnight blue sapphires,crimson red rubies and brilliant yellow sapphires,are typically,almost without exception,heat-treated or treated in some other manner to improve their appearance. In some cases this improvement is dramatic,as in the heat treatment of tanzanite,which transforms non-descript greyish crystals to vivid violet-blue stones. Buckets of plain white,cloudy corundum (sapphire) crystals can be heat treated to a clear,sparkly midnight blue. Emeralds are quite easy to identify in the field,although almost all of them have been oiled after mining to improve the clarity of the stone. Most gem crystals are difficult to identify in the field in their rough form and embedded within the hostrock,or within a bucket of placer concentrate.

SCOPE OF THIS OPEN FILE

This report is an updated version of an Open File report (Walton,1996) describing the geology and geochemistry of gemstone deposits and applicable exploration criteria. At the time of the 1996 report,emeralds had not yet been reported in the Yukon or NWT and yet it was obvious that Canada’s geology is very prospective for significant coloured gemstone deposits. Exploration efforts in the north are hampered by the short exploration season and harsh climate. The 1996 report and this update are both aimed at exploration geologists and prospectors who are unfamiliar with the geology of gemstone deposits and how to explore for them. Its objective is to provide information on gemstone deposit geology and provide general guidelines for the identification of,and exploration for,gemstone occurrences. The report focuses on a selection of some of the most valuable gemstones in the global gem trade: ruby,sapphire,emerald,chrysoberyl,tsavorite garnet,tanzanite and the gem minerals (topaz,aquamarine,tourmaline) found in pegmatites or rhyolites. Exploration criteria for other gemstones such as opal, nephrite jade,jadeite jade,zircon,lapis lazuli and rhodonite are beyond the scope of this report.

For each gem mineral covered in this report,background information is introduced and the physical and chemical characteristics of the gem are presented. The geology and genesis of each gem deposit type is summarized and exploration guidelines are listed. Because pegmatites are so closely associated with several gemstones,such as aquamarine,tourmaline and topaz,there is a chapter devoted to pegmatites and their gem minerals. Topaz rhyolite is a distinctive deposit type which is discussed in Chapter 8. Application of the exploration criteria to Yukon geology is given in Chapter 9. Reference lists are provided to those readers who want to pursue further details.

For this version of the Open File,the existing English language literature on gemstone deposits was identified through databases such as GEOREF and the Geological Survey of Canada catalog. Research was carried out on-line,through interviews and by utilizing the Geological Survey of Canada library in Ottawa,the University of British Columbia library,the main public library in Vancouver,the University of Alberta library and various magazines and newspapers. An invaluable source,as always,was the Government of the Yukon,Department of Energy,Mines and Resources library in Whitehorse,Yukon and its very knowledgeable librarians.

One of the difficulties in developing exploration guidelines is that many of the world’s gem deposits are unique. The global supply of tanzanite (the fifth largest selling coloured gemstone in the United States jewellery trade),for example,comes from one deposit in northern Tanzania. The largest and most prolific emerald deposits in the world are in Colombia; and these deposits have a unique paragenesis which is not duplicated anywhere else in the world. It is difficult to determine the range of variations and physico-chemical conditions under which a particular type of gemstone deposit may form,if there is only one deposit to study.

ABOUT COLOURED GEMSTONES

There are over 4,000 known mineral species,however,very few of these are gemstones. A gemstone can be defined as a mineral,or in the case of coral or pearl,an organic substance,which looks attractive when fashioned into an ornamental object such as a bead,carving,box,cabochon or faceted stone. A gemstone’s value is based on its beauty,rarity,durability and history behind that particular stone. Well known gemstones such as diamond,emerald,ruby and sapphire have all four of these qualities,which makes them more valuable than,for example,quartz,which is beautiful,attractive and durable,but not rare.

The carat is the standard unit of measure in the global gem trade. One carat is equal to 1/5 of a gram or 200 milligrams. The size of a one carat faceted gemstone will differ according to the specific gravity of that gem. For an easy reference,the size of an eraser at the end of a pencil is usually about one carat.

The global gem trade is divided into two broad categories: diamonds and coloured stones. (A third smaller category includes organic materials such as pearl and coral.) Unlike diamonds,there is no standard grading or pricing system in place for ruby,sapphire,emerald or other coloured stones. The price of coloured gemstones fluctuates according to supply and demand and depends on the quality and size of the gemstone rough. The price set also depends on the country from which the gemstone was mined. For instance,a quality ruby from Burma because of its superior colour,fetches a higher price than a comparable ruby from any other source. In order to take advantage of this price premium,rubies in the wholesale trade may be described by sellers as ‘Burmese,’ ‘Burma-like,’ ‘Burmese pigeon-blood red,’ etc. The stone may have actually been mined in Africa,but the dealer knows that rubies from Burma are more valuable,and this is reflected in the ‘name’ given to the stone to make it sell at a higher price.

From its discovery in some far-off land to its final destination in a jewellery store window,a gemstone’s value is marked up several times. The increase in value to size is not a one-to-one relationship,since larger coloured gemstones are more rare. Hughes (1997) describes the ‘Indian Law’ or ‘Tavernier’s Law’ formula for determining the price of a gem: 

Wt2 x C = price per stone

Table 1.1. This chart illustrates a hypothetical ‘mark-up’ of a 5-carat rough ruby mined in southeast Asia and eventually cut and sold in an American retail jewellery store. Modified from Hughes (1997)

One of the main problems in the coloured stone trade is the lack of deposits which produce a regular supply of facetable material. For example,tsavorite garnet is a beautiful gem that has never been able to fetch the higher price it deserves,simply because the supply of tsavorite is very limited. Only a handful of mines in the world provide regular supplies of good to excellent grade of coloured gemstone rough; and the jewellery industry needs to ensure that a regular supply is definitely available before the money is spent on marketing and promoting the stone.

A problem specific to Canada,and perhaps a few other countries,is the lack of awareness in the mining community about the coloured gemstone trade and its value. This lack of awareness creates problems in government departments and agencies,especially in the area of ore reserve estimation and valuation. It is hoped that this updated Open File will help promote awareness of the geological and geochemical environments in which coloured gemstones form,and that Canada will soon be known as a producer of the full spectrum of quality gemstones,from diamond to emerald,rubies,sapphires and other beautiful gems.

REFERENCES