Growth of large, high-quality hematite crystals is very difficult. The method of choice seems to be dissolution of hematite in a molten flux at high temperature followed by slow cooling. You might find Chapter 2 of this book interesting: https://books.google.com/books?id=4pbsCgAAQBAJ&printsec=frontcover#v=onepage&q&f=false

Large synthetic hematites are hard to come by, so natural crystals are often used for research. Natural crystals may have very high single crystal quality but often contain impurities such as Ti, Si, Ca, and/or Sn.

I think there are a few solution methods covered in the literature for growing smaller (i.e.