I got a bag of these in a box lot of "amplifier parts" on eBay. What are they? The only measurement I can get with my meter is 10M ohms...
As vu2nan says it is most likely a MOV (metal oxide varistor) which will have a high resistance until a large voltage is applied across it.
The exact voltage is hard to tell without a power supply that can generate a sufficient voltage; possibly above 200-300 volts, as it is often used to protect the input of AC line circuits.
Read more here: https://en.m.wikipedia.org/wiki/Varistor