Likely not a problem, you could find simmular flakes in granite but it's still granite.. They likely are formed from mineral deposites but completely petrified and inert.
Tricks you can do to determin is spray Vinigar on the rock. if it starts to sizlle an bubble it could be it leaches back into the water. If it does that, than leave it or test it any further with soaking it for a while and do some gh and kh and or ph tests to see if it altered.
Some rock for example Ardenne Calcite sizzles like mad when poured with vinigar. But than vinigar is rather agressive the rock can still be used in aqaurium because the process in water is rather slow and neglectable with doing a water change now and then we do anyway.