Famous for its vividly coloured blue Sapphires, the mines at Kanchanaburi and Bo Ploi (also spelt Bo Ploy) also produced the occasional grass green and sunflower yellow Sapphires, as well as their world famous black Spinel. The mines are situated in a jungle valley to the north west of Bangkok, the area is a very popular tourist spot and its bridge was featured in the war film “The Bridge on the River Kwai”. The main Sapphire mines are some twenty miles north of the main town. When the first discovery was made in 1918, within months thousands of artisanal miners started digging in an attempt to make their fortunes. Within a very short period, not surprisingly, the mines were all depleted.
In the late 1970s, the gem was once again rediscovered, but this time with JCBs and modern equipment mine owners were able to dig deeper. By the late 1980s they had created possibly the largest alluvial deposit Sapphire mine on the planet and reportedly they had to excavate an average of 19 tonnes of soil to uncover just one carat of gem quality Sapphire! Today, the mines are once again almost completely exhausted and very little mining is taking place, I met with one local who told me that current miners are having to sift through approximately 50 tonnes of soil to find a single piece of Sapphire.
The quality of Sapphire from the mine can vary from heavily zoned pieces, to pieces that are as open in colour and share a similar clarity to Ceylon Sapphires. In terms of tone, I have seen Bo Ploi Sapphires vary from 60% to 90%. In terms of saturation, Sapphires from the region can be amongst the best on the planet. Whilst Kashmir Sapphires are famed for their silky sleepy appearance, those from Kanchanaburi can sometimes look slightly milky.
As you will see later in this book under “clarity grading”, the GIA class Sapphires as a “type II” gemstone, meaning that they normally will feature inclusions. It is for this reason that Kanchanaburi Sapphires are nearly always heat treated. This method perfected in Thailand centuries ago benefits Sapphires from this region in two ways; it reduces the impact of the colour zoning and lessens the visual impact of the inclusions.