About 70 per cent of the world's opals are chipped out of the ground in Coober Pedy. It's no wonder this South Australian desert town is popular with Hong Kong jewellery dealers, who wander around sniffing out bargains. Many of the opal miners own shops in the town and are available to advise customers on their precious purchases. There are three things to look for when choosing an opal (Australia's national gemstone): its base colour, dominant colour and pattern. As a rule, black opal, which is a gem with a dark base, is more valuable than transparent crystal opal, which in turn is more valuable than white or milky opal. Look for red as the dominant colour, followed by orange, yellow, green and blue. Opals with a harlequin pattern, in which the colour comes in distinct patches, are more sought after than pinfire patterns, in which the colour appears as small specks. Umoona Opal Mine and Museum (14 Hutchison Street, Coober Pedy, tel: 618 8672 5288; www.umoonaopalmine.com.au ) has a comprehensive range of opal jewellery, opalised shells and opal pipes. You can spend from A$20 (HK$132) to thousands of dollars. The shop's opals come from all over Australia and include boulder opal from Queensland. Next door, the Desert Cave (below left, tel: 618 8672 5688; www.desertcave.com.au ) sells souvenirs such as opal-chip koala pins for A$5.95. You can bag an opal ring set in 18-carat gold from A$400 while prices for rings, necklaces and bracelets rise to thousands of dollars. One of the most valuable loose stones on display (large stone, below right) carries a A$120,000 price tag. The most precious stone at John and Yoka's Opal and Art (23 Hutchison Street, tel: 618 8672 3099) is the 72-carat Virgin Rainbow. Owner John Dunstan turned down an offer to exchange the stone, valued at A$1.5 million, for a property in South Australia. The store also sells opal art from A$25, rock art from A$32 and prints of paintings by 81-year-old Outback artist Harry Neumann starting at A$300. Coober Pedy Black Opal (Hutchison Street, tel: 618 8672 5861) has triplet earrings from A$75 and boulder-opal pendants from A$45. If you're looking for souvenirs try Star Opal (123 Hutchison Street, tel: 618 8672 5158) for opal pins from A$18 and rough opal pieces from 50 cents each. The store has a 28kg boulder opal valued at A$9,000. Aretas Opals (27 Hutchison Street, tel: 618 8672 5697) also has an excellent range of souvenirs and jewellery. The friendly proprietors of Discount Opalhouse (8 Hutchison Street, tel: 618 8672 5690; www.discountopalhouse.com ) do a roaring trade in gifts and souvenirs from A$10. Opal Cave (Hutchison Street, tel: 618 8672 5028; www.opalcavecooberpedy.com ) sells rough opal stones from A$50 and a range of statues of miners from A$180.