These expendable molds are made of wet sands that are used to make the mold's shape. The name comes from the fact that wet sands are used in the molding process. Green sand is not green in color, but "green" in the sense that it is used in a wet state (akin to green wood). Unlike the name suggests, "green sand" is not a type of sand on its own, but is rather a mixture of:
silica sand (SiO2), or chromite sand (FeCr2O), or zircon sand (ZrSiO4), 75 to 85%, or olivine, or staurolite, or graphite.
bentonite (clay), 5 to 11%
water, 2 to 4%
inert sludge 3 to 5%
anthracite (0 to 1%)