Ulster's Diamonds

I grew up near Donegal Town. Locals always called the open area in the middle of the town The Diamond. It was mainly used for car parking and was more of a triangle than a diamond. Streets ran from three corners

 

A map of Donegal Town from 1901.

 

 Coleraine, 1611.

Lately I've learned that there are many other town diamonds in Ulster. The centre of Ulster's plantation towns were usually given that name. The outer limits of Coleraine might vaguely suggest a diamond but its central zone, the part that is actually called The Diamond, is a rectangle. This map from 1611 shows it well. Why did the term diamond come into use?

 

Postcard of Coleraine.

Diamonds were the town's market areas and markets were central to the Plantation project. It was believed that control, regulation and commerce would help build a new society. I wonder if the hard, angular associations of the word diamond appealed, in some fundamental way, to the designers of the plantation? Still now, I feel, the word diamond suggests a nexus, lines and angles meeting in one unbreakable centre. Such a solid word was bound to appeal to those wishing to anchor themselves into the ground.