I think your dio looks great! But most old rural stone walls were 'dry' ie they didn't use mortar in between the stones, peasant farmers didn't waste money on cement. So I think maybe you should have left it the way it was.
Also as stated in a previous reply they were built from stones pulled out of the ground so they would be rougher, not round like yours.
But doing a search for some pics of some dry stone walls here in Ireland, you can find many different types of rock used, also the way the walls were built depended on the shapes of the rocks available.
Here's some different examples.
This last one is made of faily round stones, probably sourced from a beach, the wall is only one stone thick and look at those gaps!
Basically what I'm trying to say is, unless you are building a dio and depicting an stone wall thats in a photo, nobody can tell you your wall is wrong. Keep up the good work!