Rock ? ID

[bocash3]

When trying to learn new  Makro Gold Racer,  I found a  strange rock in my creek this week.

Am in an area where the original  NC Gold Rush started in the 1830's so after decades of  part time  coin hunting, I'm trying to broaden interests due to a disability.  

Makro Gold Racer read  solid 36 climbing to 37.
Garrett AT Gold  read  solid 37 bumping to 38.
Fisher  Gold Bug Pro  read  40-41.  

My minor background in geology goes back over 50 years,  but:

1. It is black
2. It is porous
3. It does not float
4. Rough texture
5. Streaks  black
6. Feels and almost weighs like Pumice, but  I think heavier.
7.  Much harder than a fingernail,  but does not appear to scratch glass.

Photos below if I can figure out how to attach them.    THANKS for any suggestion as to what you think this is.   Take care,  "Bo" 

[jasong]

Usually any vesicular igneous rock that is close to the density of pumice, but sinks in water, is classified as "scoria". It can contain different minerals, with different streak colors. Scoria is more like basalt than pumice is, so might set a detector off.

Streak isn't very meaningful when dealing with rocks instead of minerals though, in terms of ID's. 

I've encountered some vesicular material in ore shoots too which is not scoria. Not quite sure how it forms, and don't think there is really a name for it. It's usually noticeably heavier than pumice though.

Anyways, don't know, just some ideas. 

[bocash3]

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"