This boulder of actinolite is an amphibole, a double-chain inosilicate. This is a Kunz-duPont piece. The actinolite is found in North Island, New Zealand as rounded stream boulders like this one.
Actinolite crystals are long and thin.
Nephrite is a tangled, compact aggregate of actinolite crystals and is one of the forms of jade. Although actinolite is not an exceedingly hard mineral, the interlocking texture of the fibers of actinolite make nephrite jade extraordinarily tough. Everywhere it occurs, it has been used by the indigenous people for making clubs, ax blades and other tools that can resist impact. Coarse grained quartz, which is harder and takes a sharper edge, is not so tough and therefore was not so desirable to the early stone workers.