Many minerals are toxic, but only when ingested, mishandled, or improperly used. Most of the time, wearing jewelry with a gemstone/mineral, holding one, or even being near them or constant contact for extended periods of time won’t cause problems. If they did, you would already know because the toxicity would be acute. For example, quartz is toxic and can cause silicosis. But unless you inhale quartz dust, work in a quartz mine without a mask, or ingest quartz, you won’t suffer any negative effects. Another example is pyrite. Handling it is fine and won’t cause problems. But if swallowed or left in water where it dissolves easily, it could cause serious problems to living organisms. It is likely that no specimens that you’ve come across in your purchasing, mining, or discovery of minerals and gemstones are unsafe to touch or wear. So far cinnabar, arguably the most toxic mineral because of its mercury sulfide compound, is probably the most dangerous to handle and it is being barred from production and use except in very limited quantities. Mercury sulfide can be absorbed through skin, but it’s not confirmed how much is usually absorbed. It’s usually okay to handle it, but gloves are recommended.
As such, when it comes to decorating your fish tank with beautiful gemstones and dazzling minerals, extreme care must be taken. Minerals that are soluble (dissolve) in water are not too common in comparison to the total number of minerals, but those that exist have some levels of toxicity and can be fatal when ingested. Avoid dolomite, calcite, malachite, halite, and many more. This can cause fish to be killed. Most quartz crystals/stones are fine because they are not soluble in water. Acidic stones or any other types of minerals that affect pH levels are not safe. This includes limestone and to some extent sandstone. Most gemstones don’t affect acidity. Any gemstone that flakes a lot should not be placed in water. For safe measure, any ore producing mineral should not be placed in water. Some minerals have dangerous reactions when exposed to water. Pyrite, for example, can create sulfuric acid. This will definitely kill any fish you have. Pyrite is flaky and has ore anyway, so even if you aren’t trying to perform chemistry, it’s not safe regardless.
So far, a great deal of crystals and minerals would be excluded. Too many to name, so what is safe? As said, quartz crystals are safe (including amethyst, usually). Jade, granite, slate, and petrified wood are all safe. They don’t flake, have no exposed ores and aren’t metallic, they don’t dissolve in water, and aren’t toxic unless ingested. Cryptocrystalline stones are safe as well. This includes agates, jaspers, and similar stones.
None of this is concrete. PH levels, temperature, type of fish, etc. can all impact whether or not a stone is safe for your tank. It’s a vital responsibility on your part to look up whether a stone is safe for your tank or not.