What Are These?

I found these along a rail siding shallowly buried in clay soil in Portland Maine about twenty years ago. They are very hard and difficult to shatter. They do not flake well or easily although they resemble obsidian once they are broken.  I thought they might be apache tears but they look nothing like photos I have seen of other naturally occurring oval or round minerals . It looks like they might be of a volcanic substance that cooled rapidly. Perhaps they are meteorites of some kind yet they are not attracted to magnets.
I took some photographs for your perusal.

Overview of several stones. Most are smooth and have one color. I have provided close ups of the more interesting stones.

This one looks like is was broken long ago. You can see layers of color.

Most of the stones are smooth and colored in  one of various shades of black to grey and orange to buff. This one shows and interesting mix of color and texture.

This one has lovely warts.

Looks like a fruit slice.

They almost look like pictures of asteroids.

Addendum, January 10, 2013
The consensus is that they are jasper. I have read that it is possible to flake jasper to make stone tools but my efforts a few years ago to flake these things make me respect our Mousterian Neanderthal predecessors to a higher degree. These rocks are very very hard and difficult to work. Some of the stones I found might be more accurately called chert, as they are less variable and of one color. The specimens I found are almost all unbroken while most of the photographs of jasper one finds on the net are of cut specimens.

Neanderthal Jasper Flake Tool, photograph from Stone Age Artifacts.com