In article ,
"Martin H. Eastburn" wrote:

Corundum -- Al2O3 Hematite Group - Sapphire is a color type gem that isn't
red - ruby is red.
Other colors are sapphire - blue( star), black (star), yellow, ....
also known as corundum,

I'll buy that one, as I already know it's correct.

magnetite, hematite,

But not these two, which I know in both cases to be forms of iron, not
aluminum. Magnetite, in particular, is definitely ferrous - Under the
name of Lodestone, in association with a piece of string to hang it
from, or a cork and a bowl of water to float it with/on, it was
humanity's first compass needle. Except when used as a coil winding,
I've never heard of any form of aluminum exhibiting magnetic
properties... As for Hematite, I've heard it described, by several
different persons in positions to know, as "crystalized rust" because
it's essentially a lump of iron oxides with a handful of impurities.

and spinel.

Eh... Might be, but not sure, and not interested enough to find out.

Perhaps being classed in the "Hematite group" indicates they all have
similar crystal structures? They definitely don't have the same
molecular structures!

That is first year Mineralogy class. Useful when trading stones...

Mayhap someone needs to attend it...

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