Common (almandite) garnet is Fe3Al2(SiO4)3. Garnet is formed in metamorphic and some igneous rocks under conditions of high heat and relatively high pressure. While completely stable in these conditions of high heat and pressure below the Earth's surface, it is not completely stable in the conditions prevalent at the Earth's surface. If you are apt to spend time in the North Eastern USA, New England area, you will find them in the various metamorphic rocks that outcrop here. Rocks in streams etc often contain remains of these garnets and they are broken down. How long they will stay stable in an aquarium is an interesting thought. Keep us updated. The substrate looks very nice. As usual, Nikolay is innovating.The garnet is completely inert so it doesn't release anything in the water
The layout has potential - depends on the plants. What is the rock you are using? It looks fine grained, slightly weathered rounding of edges etc. The rocks seem to be a foundation on which the wood is anchored.
With a narrow (side-to-side width) aquarium, it is hard not to centre designs. Sometimes, adding elevation to the substrate on one side or other helps to alleviate the feeling of being in a narrow confine.