The Gran Sabana diamonds

The Gran Sabana is the high savanna of Bolívar State in southeastern Venezuela, the country of the tepuis, the flat-topped sandstone towers that gave Conan Doyle The Lost World. Diamonds have been coming out of its rivers since the early twentieth century, and the best known Venezuelan stone is the Barrabás, a hundred and fifty five carats, found in 1942 in the Guaniamo country.

There are two different deposits behind that. The classic workings are alluvial: diamonds concentrated in river gravels, above all around Icabarú near the Brazilian border, dug by hand and by dredge in a way that has changed less than you would expect in a century. The second is primary rock. Serious exploration at Guaniamo began in the late 1960s and in 1982 the source was found, kimberlite sheets carrying olivine and pyrope garnet, the indicator mineral that tells a geologist diamonds are possible. Guaniamo is now recognised as one of the most important diamond provinces in South America.

The honest part. Diamond country in this region has a long and well documented history of informal mining and cross-border smuggling, and it has been the subject of published investigations under exactly that heading. Venezuela's formal export position has been in and out of the international certification scheme. Add to that remote terrain, a long border, and a security situation that has been genuinely difficult for years.

So treat this as a place on the map rather than a plan. The Gran Sabana is worth knowing about because the geology is spectacular, the tepuis are among the oldest exposed rock surfaces on Earth, and the diamonds coming off them are being washed out of that ancient sandstone and its underlying shield. If you ever go, go to look at the landscape, and leave the gravels alone.