Is This A Rock?

[Jspirko]

This was in the first batch of rocks I tumbled last month.  It looks more like something crystalized over a metal object.  Any ideas?

[Trent King]

57 minutes ago, Jspirko said:

This was in the first batch of rocks I tumbled last month.  It looks more like something crystalized over a metal object.  Any ideas?

Yep it’s just a rock, looks like a granitoid type from the minerals.

[flakmagnet]

Yes, it's a rock. (Not being smart-ass, just meant it's not a meteorite or anything)

 

[jasong]

Pegmatite w/schorl or pegmatitic granite.

[afreakofnature]

Yup pegmatite.  White looks like feldspar with the more translucent minerals being quartz.  Black probably biotite maybe tourmaline can't tell in the pic.

[Jim Hemmingway]

Hi Jspirko…that's a real nice specimen you've got there. Now I’m going to step out on a limb a bit and say that your sample looks to me to be a fine example of a granitic-textured, plutonic igneous rock called diorite. You may wish to read up about it and see if your rock does match as nicely as it appears in your photo.

We come across diorite in northeastern Ontario. Its grain size is typical of other similar type rocks such as gabbro, granite and syenite. I tend to describe rocks as pegmatitic if the grain size generally exceeds about five millimeters. Diorite can vary somewhat in appearance, but as a course-grained intermediate intrusive (that is to say classified between felsic granite and mafic gabbro), it tends to be generally darker than granite. 

Diorite is typically comprised in an approximate ratio of two-thirds white, sodium-dominant plagioclase feldspar (oligoclase / andesine triclinics) to a third of amphibole such as a hornblende, or quite possibly as a biotite mica. Hence your sample has an attractive two-toned appearance similar to the example depicted below. Little or no quartz is present in diorite, otherwise we would refer to it as either a granodiorite or simply as granite just depending on how much quartz and alkali feldspar is present.

As noted on the photo, you can see that my specimen possesses no ferromagnetic strength, but does ground balance to the same elevated non-conductive range as magnetite bearing rocks and other black minerals that we encounter in the field. At GB45 the rock generates a strong negative threshold signal, again similar to mafics that are comprised of heavier materials such as magnesium and iron…………..Jim.