I am working on a worldbuiding project. In it, an advanced alien civilization traveling the galaxy find a habitable planet with conditions similar to Earth before the development of multicellular life. The aliens construct a monument on the planet, a heptagram-shaped pyramid (777 km width from tip to base, 77 km tall, area of 181,000 square km and volume of 423,000 cubic km), at the center of the largest ocean. Afterwards they return to their journey across the stars, and in the following millennia, life begins to develop on the planet (or a handwaved earth-like biosphere with humanoids develops from a colony, stargates, etc.)
Is there any material that could be used for the construction of this that would be resistant to weathering from the water and air, doesn't easily absorb water, and wouldn't be reactive with the surrounding environment? The pyramid would be completely solid and homogenous, apart from a small capstone made of a separate to-be-determined material at the top.