Usage of the phrase している


While I was working on 日本語総まとめ N3 文法, I came across the following.

ここの砂は星みたいな形 **をしている**.

Now I know that this translates to "The sand here in this area are star-
shaped."

What I don't understand here is the usage of the bold-faced phrase をしている. Can
someone please help me with this? Problem is the usage (in what sort of
occasions specifically).

