the starting point

tribe 3 progress

I start with the stone, Tribe 3 is composed of dark smooth slate
I looked it up...

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. 

 also....

slate: a dense fine-grained metamorphic rock produced by the compression of various sediments (such as clay or shale) so as to develop a characteristic cleavage

and then...

metamorphism

1. :  a change in the constitution of rock; specifically :  a pronounced change effected by pressure, heat, and water that results in a more compact and more highly crystalline condition

I had thought that slate was simply sedimentary I didn't remember the rest.

A transformation is a part of its history too.

both of these qualities suggest a part of the identity of this tribe.

The core of it/them