Chert (often called Flint when found in chalk/limestone)

Hardness: 7 (Mohs scale), Color: Dark grey/black interior with tan/cream weathered rind (cortex), Luster: Waxy to dull, Structure: Cryptocrystalline, Fracture: Conchoidal (shell-like curves)

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in sedimentary basins. The coordinates provided (near Mount Hagen, PNG) are geologically consistent with sedimentary sequences and limestone formations where chert commonly develops.

Historically used for stone tools and weapons due to its sharp conchoidal fracture. Modernly used as a road aggregate, in glassmaking, and in some decorative crafts.

Chert is so hard it can easily scratch high-quality steel. Because of its predictable breaking pattern, it was the primary choice for prehistoric hunter-gatherers to make arrowheads and knives.

Look for the sharp, curved fractures and the distinct 'rind' contrast between the interior and exterior. The location in Papua New Guinea has a rich history of stone tool production using similar localized silicates.