Bo Ploi is a sapphire mining town which lies 35 km north of Kanchanaburi and about 230 km north west of Bangkok. Blue Sapphires have been mined at Bo Ploi for over eighty years but large scale mining only really began in the mid 1980's with the arrival of modern mining techniques and improvements in heat treatment technology. At it's peak there were as much as 20 large-scale mining operations in Kanchanaburi. In 2005 only four of the larger mining companies remained. Today there is only a small amount of mining activity taking place due to high fuel costs and the present economic climate.

The alluvial deposits are spread out over a huge valley surrounded by basaltic hills. In the past the sapphires were found fairly close to the ground surface but today the JCB’s have to dig down about 20 metres to recover the gems. The soil is transported by truck to very large jigs where powerful hoses wash the soil leaving behind the gem bearing gravel. The gravel is then processed through a sieving machine and graded for size. The different sizes are taken for sorting and the sapphires are picked out by hand. The rough sapphires are then graded into faceting or cabochon qualities and sent for heat-treatment. Finally the stones are cut and polished and ready for sale. The return yield from rough to cut is about a 30%.

The quality of the sapphires recovered is mostly of commercial quality but occasionally stones of similar appearance to fine Sri Lankan sapphires are found. Of the sapphires mined in the area approximately 95% are blue with the remaining 5% being lemon yellow-green or occasionally pink. Some very large stones have been found at Bo Ploi and the mines still produce commercial stones of 10 carats and over. The only other type of gemstone found in region is black spinel or nin, as it is known locally.These spinels are often faceted, made into inexpensive jewellery and sold locally. These spinels are also found salting the parcels of dark Australian or African sapphires.

The blue sapphires are characterised by strong colour zoning and tiny particles giving the stones a milky appearance. The colour of the stones ranges from mid-blue through to fairly dark blue but not as dark as some Australian sapphires. Paler coloured stones are also occasionally found. The distinctive sleepy appearance makes these stones relatively easy to identify as Kanchanaburi sapphires. Heat treatment of Kanchanaburi blue sapphires can improve them dramatically.

At its peak the Bo Ploi deposit had the world’s largest production of blue sapphires. However, with the high costs involved in mining, it remains to be seen how much longer this deposit will continue to be economically viable. There will only be the quantity of gems that nature has provided and eventually the mines will dry up. The Bo Ploi deposits have not suffered from ecological damage sometimes associated with opencast mining. As the mining areas become exhausted, trees are planted; the ground is landscaped and some areas have been replaced with fishing parks and golf courses.