Gneiss

Hardness: 6-7 (Mohs scale); Color: Banded grey, white, tan, and dark streaks; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, splits along bands (gneissic foliation); Specific gravity: 2.6-2.9.

Formed by high-pressure, high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths like granite or shale). The bandas represent segregation of minerals during intense heat. Given the coordinates in Southwestern France (Pyrenees foothills), this likely dates to the Variscan or Hercynian orogeny (approx. 300-350 million years ago).

Used primarily in construction as crushed stone for road bases, and decorative building stone or countertops due to its durability and attractive banding.

Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada. The distinct parallel bands are called gneissic banding, which differentiates it from schist.

Identified by its characteristic alternating light and dark mineral bands. In the field near the Pyrenees, look for it in areas with exposed basement rock or riverbeds transporting material from the mountains. Collectors look for sharp, contrasting bands or included minerals like garnets.