What do you mean by “greenstone”
There are pkenty of green stones.
If by chance you mean nephrite jade,or known locally as ponamu, it has a relative density of between 2.9 to 3.03.
Even ponamu is not a good name as there are 3 varities of rock that fall under the ponamu name.
Nephrite jade-most common
Bowenite-also tangiwai-less common but if a good piece it rather special
And one other that slips my mind-least common an least valuable,like almost not worth the effort to work value.
Its really that heavy,just average density really