It looks like the largest end is a square but the smaller end - your starting end, if I understand you correctly - is not square but a long point - "diamond" you call it. What is the pattern you chose to make this? It will be good to review the mechanics the pattern you used - how it got this way to - avoid this problem in your next go-round of mitered squares
It looks like the largest end is a square because it/was still on the needles. When you stretch it out it is the same shape as the pointy end.