Limonite-stained Chert

Hardness: 6.5-7 (Mohs); Color: Rusty brown, orange, and tan; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.9.

Formed via the accumulation of silica-rich organic remains or chemical precipitation in marine environments; the brown coloration is due to the secondary oxidation and leaching of iron minerals (limonitization) during weathering. The region (Kalimantan, Borneo) features significant Cenozoic sedimentary basins where such silicates are common.

Historically used for stone tools due to sharp edges; currently used as aggregate, decorative landscaping stone, and occasionally for folk medicine or talismans in some cultures.

Chert is so durable that it often remains as 'float' in tropical riverbeds long after the surrounding limestone or bedrock has weathered away. The location (0.9°S, 113.9°E) is in Central Kalimantan, a region known for diverse sedimentary deposits and alluvial minerals.

Identify in the field by its extreme hardness (will scratch steel) and conchoidal fracture patterns. It is frequently found as rounded nodules or pebbles in stream beds across the Indonesian archipelago.