Likely serpentine (serpentinite) pebbles

The specimens appear to be rounded, polished or naturally smoothed green stones with mottled yellow-green to dark-green coloration and a dull to waxy luster. Serpentine typically has Mohs hardness of about 2.5–4, a fine-grained or fibrous crystal structure, poor to indistinct cleavage, and specific gravity around 2.5–2.6. The image alone cannot confirm the identification; similar colors can occur in jade, greenstone, or altered volcanic rock.

Serpentine forms when ultramafic rocks such as peridotite or dunite are altered by water during hydrothermal or low-temperature metamorphic processes, converting olivine and pyroxene into serpentine minerals. Serpentinite bodies commonly form in oceanic crust and along tectonic plate boundaries, especially in ophiolite belts. The exact geological age cannot be determined from the photograph and may range from Precambrian to recent tectonic settings.

Common serpentine is used as decorative stone, carvings, cabochons, tumbled stones, and ornamental objects. Massive serpentinite has also been used as a building and facing stone. Fine translucent green varieties may be sold as ornamental 'serpentine jade,' but they are generally less valuable than true jadeite or nephrite.

Serpentine is a mineral group rather than a single mineral species. Some serpentinite contains magnetite and may show weak attraction to a magnet. Chrysotile is a fibrous serpentine mineral and can occur in asbestos-bearing rocks, so unknown fibrous material should not be cut, drilled, sanded, or crushed. The yellow-green pebble may represent serpentine with iron staining or weathering.

Identification in the field is supported by a greasy or waxy feel, green mottled color, relatively low hardness, and occurrence in association with dark ultramafic rocks. A steel knife may scratch serpentine, while glass usually will not be scratched by it. A streak test, magnet test, hardness test, and thin-section or X-ray diffraction analysis would improve confidence. Common occurrences include California, Oregon, Washington, Vermont, Quebec, Italy, Switzerland, New Zealand, Afghanistan, and many ophiolite belts worldwide. The lower green pebble could also be another green altered rock, so the identification is provisional.