I ended up using the silicone and it seemed to hold alright - I was just trying to bond small river stones. I found a giant bag of them for a few bucks, so I thought I may as well try to do something with them. I've dropped it a few times and they haven't budged, so I'm assuming it will hold.
Thanks for the clarification on how long to leave them out - on the bottle it just says to rinse with water, so I rinsed it a few times and then left it soaking in a bucket for a few days. Is it just the smell that I have to get rid of?
Here's a pic of the slate - I realize I'm being paranoid about everything, but I want to get it right. My nitrate is finally rising, so hopefully not too much longer until it's cycled =)
If this is ok to use, could I dust some sand over fresh silicone to cover up the messy edges?
Thanks for the replies!