The Calcite , a calcium carbonate with formula CaCo3 and hardness of 3-4 it is found in various tone colors, due to oxide minerals element such as iron, manganese, cobalt and others. Here the green color could be caused by iron like in the yellow and red variety. Is a common material, the value is given by the polishing difficulty; it has a perfect cleavage on three rhombohedral planes and crystallized in hundreds of shapes .
Is a collectors stone only, due to the low hardness: 3 from the Mohs scale.
This yellow variety comes from Mexico and is always cloudy or with some small fractures.