Oxide of Chromium or Chromium Green discovered in the mid-19th Century and made from a metallic oxide.
A good solid mid-value green, really useful as base green for landscape painting neither really too warm or cool. Good tinting power and definitely opaque.
If you weren't mix your greens from primaries then this would be an excellent choice in the palette as it can taken in so many directions. Pigment rich.
Pigment - PG17
Vehicle - Linseed Oil
Opacity - Opaque
Drying Time - Average