Comment by red75prime

Monocrystalline silicon (silicon dioxide was my brain hiccup) seems to be sufficiently stable to last hundreds of thousands of years. [1] notices constant dissolution rate of monocrystalline silicon that will keep its rectangular shape recognizable for a long time.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5830089/ "The observation that hydrolysis of Si NMs proceeds by surface reactions only, in a predictable manner without diffusion through defects or pin-holes"