File:Wall ore obsidian.png

Here is the 4 factor test:

1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
2. the nature of the copyrighted work;
3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
4. the effect of the use upon the potential market for or value of the copyrighted work.

It may well be copyrighted, please conduct your own copyright research, and contact the copyright holder before reuse in commerce.
See also

http://www.copyright.gov/fair-use/

http://fairuse.stanford.edu/overview/fair-use/