What are some household sources of radiation detectable with a geiger counter? I recently started rock tumbling with my preschool-age kids and bought a cheap geiger counter to check out rocks we find (more from curiosity than concern). Specifically it's a GQ GMC-500Plus model with two tubes that should be able to detect beta, gamma, and x-ray.
We've watched it click for background radiation (~0.12 μSv/h) but I'd like to show the kids how it behaves in the presence of a discrete source. So far none of our rocks are radioactive. I could buy samples of uranium ore or specific isotopes for calibration, but I'm more interested in things that might already exist in my house or neighborhood.
Here's a partial list:

*

*Smoke detectors - Americium - Primarily alpha particles, probably not detectable by my counter.

*Bananas - Potassium 40 - Probably not strong enough to detect.

*Pre-1970s Fiestaware and other red/orange ceramics

*Vaseline glass

*Granite countertops

(I don't have any Fiestaware, Vaseline glass, or granite counters.)
What are some other likely household radiation sources that might be detectable with a cheap geiger counter?
 A: Frankly (and luckily...) I doubt that something useful will be suggested beyond what you already list. But depending on where you live, you could try to measure a difference in the radioactivity in your basement versus outside. If not vented properly, and somewhat deep, and in the right surrounding rock, basements can have 100s of Becquerel of activity per cubic meter of air from the radon decay chain. That gives plenty of gamma activity too. Not trivial to measure that though, you will need to average the count rate over significant time.
A: You can get thoriated tungsten electrodes from welding supply shops.
Rock samples differ considerably in their radioactivity, so try different rocks. Here in northeast Massachusetts there's quite a bit of thorium in the glacial gravel.
A: A banana doesn't contain as much potassium as most people think. An average banana has a mass of about 120 g and contains about 400 mg of potassium (about 1/12 of the USDA recommended daily intake).
By comparison, a 89 g shaker of potassium chloride*, often used as a salt substitute, contains about 43 g of potassium. So you are much more likely to detect emissions from a salt substitute (ideally pure KCl) than a banana.
* I do not endorse this product
A: Potassium carbonate or Potash  works reasonably well if you measure for some minutes. In Germany it is commonly used as baking agent, maybe you can also get it in normal stores.

A: ALL OF THEM. ALL EM frequency devices emit radiation...radio, microwave, infrared, visible light, ultraviolet, x-ray, and gamma rays, upper tiers not essentially...... but the low frequency waves by virtually all devices
