Sapphires - Processing, Grading, Characteristics, Heat Treatment and Marketing of Sapphires

Sapphires are well known gem stones. Their processing, grading, heat treatment and eventual marketing are considered with a focus on the Australian sapphire industry, in particular the industry of the New England Tablelands.

Processing

Sapphires are mechanically separated from the clay and gravel in which they lie by virtue of the former’s greater true density. Typically, the gravels are dumped into a feed box at the upper end of the plant, where they are broken up by a high pressure water jet, 25 mm in diameter and operating at a pressure of 7 kg/cm². This material is then gravity-fed over a grizzly with the undersized material (-1.5 mm) pumped to a rotating-mesh scrubbing-and-sizing trommel. The oversized material (+25 mm) is transferred to the trommel with a small backhoe.

The trommel sizes the material by allowing it to pass over progressively coarser meshes. The sized fractions pass to three two-cell jigs fitted with 2.5 mm slotted screens, which effectively trap the -5 mm, -12 mm, and -20 mm heavy minerals in trays or hutches. The pulsating action causes liquefaction of the gravel, allowing the heavy grains to sink rapidly. The undersized heavies pass through a screen at the bases of the hutches and the lights are washed over the top. Due to the clayey nature of the Strathdarr deposit, a twin-screw log washer was included in the circuit to break up the clay-rich alluvial material. The oversized material from the screen and jig tailings are deposited into a rock bin, which is taken back to the cut as backfill. Recovery rates by this process are estimated to be of the order of 90% or more. The hutches are emptied by venturi extraction at the end of the day.

Grading and Sorting for Sale

After collection of the concentrate, further processing is required to retrieve the corundum and zircon from the remaining material, which also is retained due to the nature of this processing. These remnants generally comprise paramagnetic minerals, including ilmenite, haematite, limonite, magnetite, and pleonaste. Various magnetic separators are used, ranging from simple hand-held magnets to the high technology three-pass Eriez RE-10 high intensity magnetic separator.

Following magnetic separation of the concentrate, corundum and zircon remain. The concentrate initially is sieved into different size fractions based on the pearl sieve size system, an example of which is given in Table 1. This system has allowed the standards for buying and selling to be established.

Table 1. Pearl sieve sizes.

Subsequently, the low-value zircon is removed by hand-picking, commonly known as chooking. Once the zircon is removed, the corundum is graded according to its saleability. The five general grades of corundum are given in Table 2.

Table 2. Corundum grades.

Four of the grades in Table 2 refer to blue sapphires. Parti-coloured sapphires are stones that are homogeneous green or yellow, or they may show combinations of blue, green, and/or yellow in a single stone. The New England sapphire fields, however, produce an overwhelming abundance of blue sapphires.

New technology has seen the modification of diamond sorters into colour classifiers. These machines remove the labour-intensive end of colour separation and grading of small inexpensive stones. Further work is required to perfect such machines as they correctly register only ~90% of the stones examined. The American company Gem Technology Systems may provide advancement in this area with their highly innovative machinery, which was introduced in 1992. They currently are developing rough colour grading machines, which should surpass the existing colour sorters.

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Characteristics of Sapphire

Table 3 lists some of the characteristics that result in the desired wear and colour properties of sapphire as a gem material.

Table 3. Characteristics of sapphire.

The hardness of sapphire results in its being highly sought after for incorporation into jewellery. However, the main attraction of sapphires, in particular those from the New England district, are the desirable blue colours found. Colours are graded into categories ranging from Type A to Type D.

Pure sapphire is colourless. In the presence of the common impurities iron and titanium, the blue colour is generated by the Fe-Ti charge-transfer. Chemical analyses of New England blue sapphires are given in Table 4. These data were obtained using an electron microprobe (Cameca SX-50) on stones from Reddestone Creek No. 1 Mine.

Table 4. Quantitative analysis (weight%)

Heat Treatment

Today, heat treatment is an integral part of the sapphire trade, with this process being widely practised and accepted throughout the world. In fact, the practice is so widespread that sapphires that are indicated not to have been heat treated are questioned. The use of heat treatment has been responsible for many of the lower-quality sapphires being sold for higher prices than would otherwise be possible, thus prolonging the economic life of many mines. Many of the New England sapphires possess a characteristic trait, which decreases their value, in the form of needle-like rutile (TiO₂) precipitates, commonly known as silk.

Heat treatment involves two aspects of enhancement, these being colour modification and clarity improvement. Both shortcomings can be improved concurrently by high-temperature (~1600°C) reduction. The blue colour can be intensified and homogenised by reduction of Fe³⁺, solution of Ti⁴⁺, and the associated charge-transfer. The clarity is improved by reduction of the TiO₂ to Ti₂O₃, the latter of which is more soluble in the Al₂O₃ lattice. The relevant reaction is:

4TiO₂ + Fe₂0₃ + 3H₂ --> 2Ti₂0₃ + 2FeO + 3H₂0

Marketing

Today, correct marketing of the entire run-of-mine material is extremely important. The miner must sell the lower quality material as well as the premium grade sapphire to make the mining of any deposit viable. At the turn of the century, the best Australian sapphires were sold to Germany, where the large cutting, polishing, and jewellery industries were based. The 1960s brought a significant shift in the perception of business people from Southeast Asia, particularly those from Thailand, who saw the benefits of cutting the lower quality material for the cheaper end of the jewellery market. The considerably lower labour costs within their own countries in comparison to those in Europe saw the growth of the industry within Asian countries, This shift, further aided by government support and the discovery of new sapphire and ruby deposits, has allowed Thailand to develop fully while consolidating its present virtual monopoly in the gem and jewellery industries. The actual cutting and polishing of the rough sapphire are highly labour intensive. Thailand claims to employ 600,000 cutters, and the labour costs of cutting and polishing in this country are estimated to be between 5 to 10% of those required in Australia. To date, automatic cutting machines waste up to 90% of the rough stone whereas the average lapidary cutter would lose only 65 %. Table 5 summarises the current labour costs of lapidaries from various countries.

Table 5. Lapidary labour costs per person per day.

The sapphires are sold in two main ways, these being through contracts and by spot sales. Contracts, the more desirable method, are usually six monthly, whereby an agreement is drawn up for the miner to sell all run-of-mine production to one buyer at an agreed price. At the end of each six months, new contracts are established. The less favourable way is to sell the production to travelling buyers who pass through the New England district, negotiating the best price.

Summary and Conclusions

For the future of the New England sapphire industry to advance, a constructive blend of the working miners’ knowledge and an acceptance of new technology must be found if they are to obtain the maximum benefit. There are four main difficulties facing Australian sapphire miners: new finds of Australian alluvial deposits are becoming rarer, current heat treatment technology is not sufficiently developed, third world labour is very inexpensive, and new overseas sapphire deposits have been located recently. In order to maintain its dominance of the world trade in sapphire rough, New England miners must make advances in detailed exploration and highly economical downstream processing. The latter will become increasingly important if a higher percentage of the final added value is to be retained.

Note: A complete list of references can be obtained by referring to the original text.