Is a diamond a polygon?

Could someone please clarify for me whether a diamond, in its most basic form as a gemstone, can actually be classified as a polygon? My understanding of polygons is that they are two-dimensional shapes with straight sides and angles, whereas diamonds are three-dimensional crystalline structures. Is there perhaps a specific context or definition of diamond that I'm missing, where it could somehow be considered a polygon? I'm genuinely curious about this and would appreciate any clarification on the matter.