Warning, cummingtonite is less exciting than you think unless you are passionate about mineralogy. First discovered in Cummington, Massachusetts in 1824, this beautiful mineral exists in various colours: translucent dark green, grey, brown and colourless.
In more scientific terms, it's an amphibole mineral belonging to the magnesium-iron-manganese subgroup, which occurs as lamellae and fibres in metamorphic rocks. The chemical composition of the cummingtonite is {Mg2}{Mg5}(Si8O22)(OH)2.
If you want to venture out in the wild to find this mineral, some locations include the Birkeland mine in Rogaland (Norway), the Franklin Mine in New Jersey (USA) and the Jackson County Iron Mine in Wisconsin (USA).