As far as I know, this is an original design (at least, I’ve not found anything like it). The diamond is actually a simple variation on the traditional granny square.

The vartations are:

1. Use 4dc clusters instead of 3dc clusters
2. Put 2 ch between all clusters along edge
3. The 1st and 3rd corners are made up of (4 dc, ch 2, 4 dc)
4. The 2nd and 4th corners are made jup of (4dc, ch 7, 4dc)

The pattern gives specific instuctions for creating the motif.

Although the instructions are written so that you crochet-join-as-you-go (CJAYG) using a flat braid, the motifs can be completed and joined with any method you desire.

Placement of motifs accorging to diagram is critical to get the optical Illusion effect.