You can use epoxy putty. They sell it at
lfs and online. it is a green tube with a white center. You break some off and mix it then stick it to what you want. If you dont use a lot you wont see it. If you want to, you could try to wedge the rock or try to use other small rocks to hold it down. If that doesnt work then the epoxy will work.
HoldFast Epoxy Stick
This an example. They have another brand as well.