Is wearing hijab a farz?

I wear a hijab but some of my muslim friends don't. So I constantly hear questions like why do you wear it and she doesn't. I tell them that we are required to wear it but they don't so they are commiting a sin. But those who don't wear it say that it's really optional and depends on the person whether to wear it or not. So who's right? And what are some verses from the Qu'ran which would support either answers? I mean, if it is a farz then I would like to know some Ayaths (verses) which would support me. Thank you.