Hi mate, there are a number of combinations that work and yes generally the drill is harder than the hearth, but on some occasions you can find the same wood being used for both.
Ash drill on a Willow hearth
Cattail drill on a limewood hearth.
Elder drill on an Elder hearth.
Elder drill and a Willow hearth.
Elder drill on a Pine hearth.
Elder drill on a clematis hearth.
Hazel drill on a willow hearth.
Hazel drill on Ivy hearth.
Hazel drill on a Crack Willow hearth.
Hazel drill on a Limewood hearth.
Hazel drill on a Pine hearth.
Hazel drill on a Sycamore hearth.
Ivy drill on a Ivy hearth.
Sycamore drill on Ivy hearth.
Ivy drill on an Alder hearth.
Poplar, Limewood and Cedar should work just fine on a cedar hearth board.
Sycamore drill on a Sycamore hearth.
Willow drill and hearth.
This was a list from on the forum back in 2004, it's by no means complete as there are gonna hundreds of other combinations as well, but it's a start.