世界上有甚麽結構是最堅固的?

Stone Arch

Most of the ancient building / architecture are built by stone (or marble) arch

http://www.technologystudent.com/struct1/arch1.htm
http://chestofbooks.com/architecture/Building-Construction-V2/Stone-Arches.html

something like diamond,
every molecules attach to 4 others.

金字塔

Pyramid is the most stable and most durable structure in the world. Except some erosion upon weathering, it is comparatively the most intact building heritage we can find.