Well, based on high school biology from 28 years ago, and what I was awake for, Isn't a substantial amount of what plants are made from come from the soil?
So what you do is you have your genetically or magically altered plants incorporate Granite or maybe limestone or basalt from volcanic rock into the cell wall structure?
Volcanic rock and soil are great for growing Bonsai plants in. The plants take in the soil in greater proportion and form a basalt formation in the cell walls to create a wood that has a lot of the same properties as Basalt.
LImestone is even easier. Suppose it takes in CO2, uses part of it for photosynthesis and uses the rest of the carbon to form CaCO3, or Calcium Carbonate to use in the cell wall structure.
I'm not so sure about granite since it is a heat and pressure kind of igneous rock. I cant think of how to hand wave the required amounts of mica, feldspar, and quartz.
Here is why I think having the plant incorporate the substance into the cell walls is important. If you have ever looked through a microscope at some onion, it kinda looks like a brick wall. Onions grow in layers. something else reminds me of a brick wall and has layers: Timbrel Vaulting. Timbrel vaulting is very strong for it's mass and thickness.
Combine drawing minerals from the soil and incorporating them into cell walls with a plausible structure and you get plants as strong, if not stronger, than stone.
I think growing stone could be fun