Jasong: I think you hit the nail:
My area of NC was supposed to have had volcanoes over 500 mya. So how this rock got
exposed really has my curiosity.
I do remember "Scoria" from college geology over 50 years ago.
AND this one "fits." Too hard for fingernail but in spite of fact I Thought it would scratch
glass, it did not. Seems that glass has a hardness of about 6.5 if I remember...................
So here is what an old science book said: (and the "magnetite" would account
for it annoying the detector.)
Scoria is a volcanic igneous rock. Also referred to as scoriaceous basalt, a term commonly used to indicate a basaltic pumice. It is commonly composed of approximately 50% silica and 10% calcium oxide with lesser contents of potash and soda. It is an extrusive igneous rock whose major minerals are plagioclase, pyroxene and olivine. Minor mneral contents may include apatite, biotite, hematite, hornblende, ilmenite, magnetite, and quartz.
It has a relative hardness of 5-6.
Thanks so much! With a science/biology background, I still love learning! Take care, "Bo"