Most likely 5ft, still a possibility I'm not 100% on what it is as there are quite a few types of Iron Spinels. The Spinel from say Grabben Gullen is the typical Black Spinel which is a fantastic jet black gemstone which is Magnesium Aluminium Oxide MgAl2O4, looks awesome cut and polished, is non-magnetic. A lot of the Spinel from Nth NSW I've seen is very silvery looking and very magnetic, I've got some here from BB.
Check the link as it's not a bad read on the subject.
https://en.wikipedia.org/wiki/Spinel_group