Since a floating shelf needs a batton or something for mounting the only time I have done something is by creating an opening with melamine panels either side & at the back of the space. Then you can create a slot to slide the stone piece in and use some melamine capping at the front to make it look like the stone is using concealed fixings.
Sounds quite technical to achieve depending on the area you're placing it.
A follow-up to my last comment... If your stone mason loves you enough you could get them to make a 3 sided stone clad shelf, with a miter return for the front and then bottom piece of the shelf. Depending on the thickness of your stone, this would create a cavity that you could actually sleeve over a baton or concealed wall fixings so that you have a visibly floating piece of stone.
GreenHaus Design
GreenHaus Design
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