I was given a great lead by another friend who told me it looked like a Botryoidal sarsen stone, which it does. Only problem is that sarsen stones aren't associated with the US. But that at least put me on the right track for searching other Botryoidal stones. This has been great fun over the past day or so, as I'm definitely getting a better education in geology - which I've always loved anyway. :-)

Here's the text regarding the properties of the specimen:

Found specimen on the top of a sandstone cliff in Inyo County, California, where it was fairly evident that other lava (igneous) rocks had blown in from the eruption of a long-ago, far away volcano. This specimen, however, looked nothing like any of the other rocks on the surface. I've spent most of the morning researching igneous rocks, ...as well as all other types of rocks, minerals and meteorites, and have found nothing that matches what this rock looks like (or acts like). It weighs 14.04 lbs (and it actually surprises everyone who picks it up by how heavy it is in comparison to its size). It displaced approximately 82 fluid ounces (though measurements are not very exact in my kitchen). It also pushes a compass 5-30 degrees off course when the compass is placed upon it, but magnets do not stick to it.

Additional testing done:
Scratch Test: it scratches ceramic tile but is scratched by aluminum oxide
Streak Test: it leaves a reddish brown streak when rubbed on abrasive surface
Acid Test: it does not bubble when weak acid is applied
Luster: dull/earthy
Cleavage: does not break, so can't tell yet
Density/Gravity: 2.62 grams/cubic cm
Additional appearance notes: there are miniscule "sparkly" dots that are visible on the surface, but don't follow a pattern and are more intermittent in frequency. It also "pings" or "tings" when tapped lightly.