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The diamond-bearing deposits in the south of the island, namely, in the districts of Tanah-Laut, Martapura, and Riam are of recent formation, and overlie Eocene strata in the same way as those described above. These Tertiary strata, which in places include thin beds of coal, rest on ancient crystalline rocks, such as mica-schist, chlorite-schist, talc-schist, and homblende-schist, and like these are inclined and faulted. Interbedded with the Tertiary strata, and specially towards their base, are sheets of recent eruptive rocks (andesites). The diamond-bearing deposits form a broad band round the seaward slopes of the Tertiary hills, while the gold-sands of the region, which contain no diamonds, rest on ancient schists. The actual diamantiferous stratum is constituted of more or less rounded pebbles of various minerals and of sand, either loose or bound together by clay. The mineral most abundantly present is quartz of various colors, after which come fragments of andesite and micaceous sandstone. A blue mineral, formerly thought to be quartz but which has now been proved to be corundum (sapphire), though of no value as a gem has yet a certain importance as an indication of the nature of the deposit. It occurs in the same manner as at Landak, and its presence is regarded by miners as indicating the existence of diamonds in the deposit; it is only after they are satisfied as to the presence of the blue mineral that they apply themselves to an exhaustive search for diamonds.

As a rule, the diamonds are found lying singly and loose in the gravel; sometimes, however, they are cemented by limonite to a pebble or rock-fragment. They are often accompanied by scales of gold and platinum and by grains of chromite and magnetite. The thickness of the diamond-bearing bed varies between 20 centimeters and 2 meters, beds of the greater thickness being usually found filling up depressions in the surface of the ground. The bed rests on a blue clay and is overlain by 1 to 6 meters of gravel and sand, and sometimes, as in the neighborhood of Bentok, by a layer of nodules of limonite. This diamond-bearing stratum is found mainly in the neighborhood of rivers and in surface depressions, which in the rainy season become filled with water.

The mining methods adopted here by the Malays appeared to be the same as those practiced at Landak; the workings were mainly in the neighborhood of the village of Tjampaka, in the district of Martapura, where in the year 1868 stones to the value of £1250 had been found; also near Banju-Irang, Bentok, and Liang Angang, all in the district of Tanah-Laut. Thousands of small mines were seen in these districts; the majority of them, however, are now abandoned, for since the great fall in the price of diamonds in 1878, the miners can easily obtain more lucrative employment in the gold mines, tea plantations, etc. Franco-Dutch Company in 1882 obtained the concession of a stretch of country of 2,000 hectares that is about eight square miles in area, for a period of twenty-five years, with the purpose of diamond mining. On this area, which lies between Tjampaka and Banju-Irang, machinery was set up, but in the very next year, 1883, the work was abandoned, and up to the present has not been recommenced. From the above accounts it is clear that the production of the whole southern group is quite insignificant.

Diamonds are also found in the Kusan district, which lies between the rivers Danau and Wauwan in the State of Pegattan, this latter being a Dutch dependency in East Borneo. The stones are of good quality, but here also the yield is poor.

It is impossible to arrive at anything more than an approximate estimate of the production of diamonds iii Borneo. From an early date the Malay princes assumed the right of appropriating at a fixed price all stones exceeding 5 carats in weight found in their own dominions. Thus these stones never leave the country, and no record of their occurrence is kept. In the table given below will be found a few returns published by the Dutch Government in carats and Dutch florins of diamonds imported into Java.