I think the idea about gemstones is a really good one. The tower also definitely needs some sort of an entrance to get to the top of it, so yeah, a door would be good. Maybe some pipes and cables. If it still looks rather plain, I'd suggest painting on some runes on the sides.
_________________
Siticus Empyrean Vision - my Facebook page with various painting projects
Siticus' Empyrean Visions log - the project log for my Aeldari works and beyond