TITLE: How to measure the volume of rock?
QUESTION [6 upvotes]: I have a object which is similar to the shape of irregular rock like this 
I would like to find the volume of this. How to do it?
If I have to find the volume, what are the things I would need. eg., If it is cylindrical, I would measure length and diameter. But, it is irregularly shaped. Like the above rock. 
Where should I start? Couple of google search says something related to integration and contours. Somebody pls give me some handle :) I would say i'm very beginner level in math.
Many Thanks :)
Edit:
60 to 70% accuracy would be helpful.

REPLY [3 votes]: There is a very simple and elegant algorithm for calculating the volume of an arbitrary, closed mesh described in this paper (Zhang, Cha, and Tsuhan Chen. "Efficient feature extraction for 2D/3D objects in mesh representation." Image Processing, 2001. Proceedings. 2001 International Conference on. Vol. 3. IEEE, 2001. Page 2). The trick is to base a tetrahedron on each triangle and top it off at the origin and then sum up all volumes, signed by whether the triangle faces the origin.