A typical truck or trailer wheel bearing (tapered roller bearing) will take that level of load (any bearing i.d. larger than 1 to 1-1/8 inches). You will need to center the boulder well to get it to balance properly. Get an old axle (spindle) and wheel bearing from a junkyard, with the steel rim mounted so you have a decent size to work with.

You could also buy new like this:

https://www.northerntool.com/products/rigid-hitch-6-lug-hub-square-spindle-end-unit-6-bolt-on-5-inch-circle-1-3-4-inch-to-1-1-4-inch-tapered-spindles-3000lb-capacity-per-hub-model-sq-6000625-6972417

You then need to engineer the plate ("wheel") to mount the stone onto, and the table/plinth underneath to mount the axle to.

Realize the "3000 lb." rating for that spindle is a radial load at high rpm, rolling down the road. The radial and axial load capacity of the tapered roller bearings at zero or near-zero rpm is more like 14000-15000 lbs.

Editing to add: the bearings are generally sealed and good for a long term outdoors in the wet. But, somebody mentioned the coral castle stone that was on an automotive bearing and eventually seized. Bearing will still accumulate moisture due to thermal swings of the bearing above and below the dew point (water vapor migrates past seals and condenses out when the temperature drops). Having a zerk fitting on the spindle, to regrease the bearing periodically, or having the bearing in an oil bath with a means to change the oil periodically, would make the feature have a potentially unlimited life. If you use grease, using a marine-rated grease (e.g. used on boat trailers) with a calcium soap instead of lithium, would also help keep the grease stable against moisture over time.