Chert (or possibly Flint)

Hardness: 6.5-7 on Mohs scale; Color: Variable, commonly dark gray, black, brown, green, or red, often with lighter colored bands or mottling; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None; exhibits conchoidal fracture (smooth, curved breaks resembling shell fragments); Specific Gravity: 2.58-2.65

Chert forms from the accumulation of silica-rich skeletal remains of microscopic marine organisms (like diatoms, radiolarians, and sponges) on the seafloor, or from the diagenetic alteration of silica gel. Over time, these silica deposits compact and recrystallize into dense chert nodules or beds within other sedimentary rocks like limestone or chalk. Geological age varies widely, from Precambrian to Cenozoic, depending on the specific deposit.

Historically, due to its hardness and conchoidal fracture, chert (especially flint) was extensively used by prehistoric cultures for making tools, weapons (arrowheads, spear points), and for starting fires (striking against steel). Today, chert is used as an aggregate in construction materials, in road beds, and sometimes as an abrasive. Attractive varieties can be polished for decorative items or cabochons, though it's less common in fine jewelry.

Flint is a dark, fine-grained variety of chert. The terms are often used interchangeably, although 'flint' typically refers to the high-quality, often black or grey chert found in chalk deposits. The striking of flint against steel was a common method for starting fires until modern match technology. Chert nodules often form irregular shapes within host rocks.

In the field, chert can be identified by its distinctive conchoidal fracture, its hardness (it can scratch glass and steel), and its waxy to dull luster. It often occurs as nodules or layers within limestone and chalk formations. Look for sharp, curving edges when broken. Common locations include various sedimentary basins worldwide, particularly where ancient marine environments were rich in silica-secreting organisms. Good places to find it are in bedrock exposures of limestone or chalk, and in riverbeds or gravel pits where it has been eroded from these formations. Collectors often look for colorful or uniquely shaped specimens, and prehistoric artifacts made of chert.