There are rocks with natural ingredients like arsenic, iron, lead, etc. It isn't hard to get a field guide and identify them,
Many rocks will dissolve and the minerals may cause some fish from mineral poor regions serious problems. A fish is tuned to deal with keeping water out of its body, and the wrong mineral content can throw those physical systems out of whack.
Most rocks are fine, and all my tanks are decorated with basalt and granite I pick up from local beaches.
Water holds dissolved minerals, and what minerals and how many depend on what rocks it's in contact with.
In the Amazon, the rocks don't release minerals, and the hardness (mineral measure) of the water is very low. Since the kidneys of a fish work hard to keep water out, evolution tunes them to deal with the mineral content they live in. It is a question of water density, really, and of water purity.
In Mexico, I've caught mollies in water surrounded by limestone, a rock that constantly releases minerals. If you put a molly in Amazon (soft) water, its kidneys can't cope with the different pressure, and it gradually dies, often after neurological problems with shimmying (swimming in one spot but not going anywhere).
It's really important to do your homework and choose fish that thrive in the water you have. Where I live, the water comes from a series of forest lakes. if you dip a glass in the lake, the water before treatment is stained by tannins - from plants decomposing. It's clear after treatment, but the mineral content is very low, and my rainforest fish species love it.
Generally, the pH (if it's new, google it) is acid in soft water and alkaline in hard water. Water is very complicated stuff, and aquarists have to learn the simple basics to succeed longterm.
Most rocks found in creek and river beds will be OK because they have been exposed to the water and will have leached most of any possible contaminants, emphasis on most you still have to check them. Typically you will want to stay away from sedimentary rocks, they often are cemented with carbonates, carbonates can affect your pH, KH, and GH. You will also want to stay away from anything with blue or green staining as these usually have copper in them. Finally stay away from anything with metallic crystals. Again, you will typically not find the bad rocks, except shale and limestone in creeks.
Metamorphic rocks are often good rocks to use, with slate being the most layered and gneiss on the other side almost like granite (there are a whole range of metamorphic rocks). Slate and Gneiss used to be common in parts of the UK, some very nice Blue Gneiss can be found in Scotland. Ordinary igneous rocks like lavas, granites, etc. are typically good too.
Limestone is common in the UK and can be found in unique shapes and forms, but it is only suitable for higher pH, and KH type tanks with fish appropriate for those conditions.
The major issue with DYI rocks is the introduction of carbonates which can cause the pH, and KH to rise. OK if that is what you want but disastrous for an acidic soft water tank. You can test for carbonates by dropping vinegar on the rocks and looking for bubbling. If there is bubbling there are carbonates.
As others have suggested a little research on the rocks in your area will make collecting more productive.
That unit, Degrees Clarke, is not one of the two used in fish keeping, but the calculator on here can convert it to the units we do use.
Your hardness is 6.3 dH and 112 ppm. Fish profiles will use one or other of those units.
UK water companies always make it sound harder than it really is. You have soft water; hard water fish would suffer in that hardness.
You need to make sure any rocks you find will not increase hardness, Uberhoust has told you how to tell
If you put it underwater and then rub it with a finger and it makes a cloud? You don't want that rock.
Exceptions might be limestone for fish that like that it can dissolve. Otherwise avoid fieldstones and the like.