Could these stones be nephrite Jade? How to know?

[Greenbird]

I have found these stones at the beach, they seem to be super polished by the sea waves for long time. I have collected so much seaglass over the years and they don't look or feel like any kind of glass found before. I've been told they could be nephrite Jade but I am not sure. They feel heave and cold in the hand, but when scratched with metal tip, I can see a slight scratch behind. They look opaque black and only green when a torch is placed behind. Very soft to the touch and shiny black after touching. I found them in Spain.