I don't think using this auction house as a reference to imply your item is an authentic native American spade is legitimate.
Artemis Gallery states "Artemis Gallery specializes in the sale of antiquities, ancient and ethnographic art, and we sell only authentic, legally-acquired examples. "
However, in browsing their past auction I noted they have lots from many places throughout the world, with this the only "native American" relic I seen.
I noted many lots touting a COA, but I did not find a single COA reference backed up with an image of the COA or details on who provided the COA. So, the referenced COA could have been printed by Micky Mouse for all I know reading the lot details.
The item you referenced MAY be an authentic native American spade. It has the correct shape but authenticating an item from a photo is tenuous at best. That said a bidder would have to rely on the auction house experience in the field, and it appears they have little in authenticating native American relics. Like I said, they can claim just about anything. I can say my dog can locate relics, but can he really?
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Note that the offered lot is much more defined and symmetric than the specimen you are displaying. This does appear to be flint. The majority of authentic spade, notched hoes and the like are mill creek chert. I cannot say if this hoe is made from mill creek.
Another point is there are auction houses which specialize in native American relics. Artemis Gallery appears to be generic antique focused.
I am still on the fence concerning your specimen. Leaning more towards natural.