Greenstone / Greywacke

Hardness 6-7 Mohs; Color: Dull grayish-green; Luster: Matte to earthy; Structure: Fine-grained, microcrystalline; Cleavage: None; Specific Gravity: 2.6-2.9

Formed through low-grade regional metamorphism of clay-rich sandstones (greywackes) under high pressure and low temperature conditions, typical of subduction zones and accretionary wedges.

Commonly used as river rock for landscaping, construction aggregate, and occasionally by lapidaries for polishing into smooth worrying stones.

The greenish tint is primarily due to the presence of chlorite, epidote, or actinolite minerals formed during the metamorphic process. In some cultures, particularly the Māori of New Zealand, specific high-quality greenstones (Pounamu) hold immense cultural and spiritual value.

Identify in the field by its characteristic 'dirty' green-grey color and smooth, water-worn texture when found in riverbeds or beaches. It is much harder and denser than typical shale or siltstone.