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
Why did you take that down? Was it just design considerations or something to do with the substrate? Why did you change the substrate? Just testing?* Active carbon on the bottom.
*Peat/pumice/laterite/lava rock as a second layer.