[Osmium]

This infographic does a nice job showing which are from the struts and which from the mirrors: https://webbtelescope.org/contents/media/images/01G529MX46J7...

Of course, the JWST without its struts and its mirrors wouldn't be much of a telescope! So the diffraction spikes are a part of doing business and also have the helpful side effect of allowing us to quickly know which objects are stars vs galaxies, since only the former have the really prominent spikes.