1 gem = 25000 building blocks.
There is no reason to buy building blocks unless you are gifting them.
So you can purchase a Tier 6 building with 50 gems or 1250000 building blocks or any combination of the two.
I find it easiest to just think of cost of buildings in terms of gems.
I wrote more about it here: New Buildings, July 2025 - #13 by Elanor