Wow! First time one of my scenarios reached 100 subs! Thanks all, and hope you enjoyed it!

To make perfect circles start a building with a post or pole that is centered on the grid (its position does not change when rotated). I placed a gem 5m from the pole. Copied the pole and gem and rotated 180 degrees. Now selecting and copying only the two gems and rotating gives perfect rotation. Carefully align, select all four gems, copy and rotate. Repeat with eight gems, etc. to build a circle. Select circle, copy, elevate, rotate and align. At the end, delete the pole, cut out half the gems and flip the building on its side.

I'm curious how the Gemstone Arch is so neat