Real shale for ballast? Wow...

Haven't worked with shale outside of a few shingles, so I'm no expert on its electrical properties, but I think it's not going to be very conductive until you get to temperatures where your house has already caught fire (900 degrees C was cited in one instance).  Since a cubic foot of broken shale weighs about 99 pounds, comparable to sand, you're probably going to be OK if your benchwork is reasonably robust.

Janet N.

crushed shale.?

Does it crush into something resembling scale prototype ballast?   .....DaveB  

sharp edges?

Does shale flake, like flint, resulting in tiny pieces with sharp edges or points or is crushed shale like sand?

If it's free or nearly so, get a bucket and test it! Test for conductivity with a multimeter and use a magnet to test for any impurities with magnetic properties.