Hi, can I put stones off the beach in a goldfish bowl if they've been sterilised?
What about quartz cristals are they ok for goldfish?
The most important criteria is how hard the stones are.
They should be marble or granite, not sandstone as they would dissolve over time releasing whatever chemicals they have absorbed into the aquarium water.
Sea shells are another example of this. Even though they feel hard to the touch, they slowly dissolve, releasing calcium into the water, making it alkaline.
Quartz crystals won't dissolve, but are you sure there are no heavy metals that will be released into the water? Fish are very sensitive to heavy metals.
The other consideration is are the crystals smooth or rough? This question also applies to your sea stones.
Fancy Goldfish especially can hurt themselves on rough surfaces in their aquarium.