What are some NON-Conductive rocks that I can put in my computer tank submerged in mineral oil?

Most rocks are non-conductive. You can use almost anything non-metallic. Granite would be a good choice, so would quartz, sandstone, limestone, marble. You want to stay away from iron ores and things like pyrite. Radio Shack has very inexpensive ohm meters. If you're not sure you can get one of these and touch the probes to your rock. If the meter moves at all you can't use it. But it will be very rare to see the meter move.