The overall gem value of your city is what influences what you get in your treasure boxes. The initial days after a purchase (the HMP) comes with a bonus to the value of the building so you should see that in your treasure boxes.
Personally I prefer the look of more of the buildings than the monuments, and to evolve monuments using buildings rather than other monuments because at Level 1 to 2, and Level 2 to 3 you can do it with 3 buildings if you have the right value ones and then your overall city gem value remains the same but you have lost fewer buildings. Each to our own eh!
Also, don’t forget what Elanor reminded us all off the other day One free gem daily - #2 by Elanor
and the weekly free coins every Tuesday