I was given and entire box filled with these. Some weighing up to 15 pounds. Can anyone tell me anything about these and what they are called? Are they worth anything? What are they generally used for? The back of the one in the picture actually looks and feels like tree bark. But is as hard as stone and really heavy. And the inside of it is different colors.

Sorry the pictures are so low quality. Had to take them with my cell. /:
http://i53.tinypic.com/14twy9f.jpg
http://i54.tinypic.com/2e51pc2.jpg

I think that possibly the second one may be a petrified tree.

Stringer

I agree.
They are collected. Sell them on eBay.

Check the colors of this Petrified wood, Nice.

http://www.legendsofamerica.com/photos-arizona/PetrifiedWood.jpg

At the least, keep a nice sample for yourself

Some auctions on eBay with "Petrified Wood":

http://collectibles.shop.ebay.com/Rocks-Fossils-Minerals-/3213/i.html?_nkw=petrified+wood&_catref=1&_fln=1&_trksid=p3286.c0.m282