Native Rocks

[mwhite4823]

I have some colorful rocks dug out of my yard in upper Montgomery County, MD. They appear to be a granite, but they have blue and green iridescent areas. I thought it might be from lichen, but they remain after I boiled them for 1 hr. I am concerned that the colors might come from copper and poison the fish. Does anyone know about these rocks or have any experience in their aquarium with them?

[Jordanella]

As a retired geologist, I very much recommend you do not use the rock! Your area has complex geology so I cannot be sure what you have, but what many people ID as granite isn't. Also, the iridescence is almost certainly not due to lichens.

If you want to send me a sample, I will ID it for you. Something half the size of a book of paper matches is usually enough for me to detyermine what it is.

There *are* deposits of metal ores in the area, including an ore of chromium, so be careful!

[SCMurphy]

I'd like to see these rocks too. Maybe I can get you to drop one off, and take some plants home with you.

I can have a geologist friend of mine take a look and you'll have 2 opinions.

I hope the opinions are the same, I'm just setting myself up for conflict here.