Trying to fix the diamond material

I have spent about an hour trying to figure out why my diamonds in one file look completely different in another file:

The glove shape still isn’t finished, it’s just a start on the basic shape.

Can you see in the first image the diamond on the right looks like a diamond, albeit a misshapened one? (I thought ‘misshapened’ was a word? My computer says no)

As you scroll down you’ll see the other side of the two diamonds.

I’ve tried following the tutorial again where I learned how to make the diamond ‘material’ and it still doesn’t work. Perhaps it is a setting that should be different? I’m not sure but I’ll have to keep trying.