Should be fine as long as it's pure (naturally found crystals often have other minerals through them). I would rinse/soak it a while first, but I know even a lot of gravel is quartz based.

 

quartz is silica based with iron making up its pigment. it will be fine as long as it isn't sharp and doesn't contain any metallic veins. also do not pour boiling water on quartz or try sterilizing it in the oven as it will explode or shatter. even if the iron leaches out it will be in nominal amounts.

 

As a geologist, yes it's fine as long as it is just the crystal structure (i.e. it is not attached to a host rock).

Also regarding pouring boiling water or sterilizing it in the oven, it won't explode. You will just ruin the colour (heat treatment)

The iron/manganese/titanium will not leech out because it is part of the crystal structure on a molecular level.

TL;DR just rinse it off in some warm water and it will be fine.

 

sorry i didn't mean explode but rather crack. this happened to mine.

 

My mother had a chunk of pink quartz in her aquarium for years and never had a problem.

 

a tank we just started has rose quartz, amethyst and obsidian pieces (tumbled obsidian, of course!) and from all of our research, the three are all fine to use. pretty much any quartz or volcanic-origin gemstone is aquarium safe.