A bit more detail on what you mean by pretty hard rocky soil would be helpful. Rocky soil can take lots of different forms. This is what I deal with in hills of NC. When I get into this stuff then ONLY carbide tipped tip and carbide bullets on the first flight and then hard face on the flight edge for next foot.
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If your talking small bank run gravel rocks then forged spade teeth ( Pengo / Esco ) will dig well. Depending on how your existing teeth are mounted to your existing post hole auger will determine what your choice for teeth are UNLESS your willing to replace the root and the tooth with preferred item.
The choices for teeth are huge so have fun googling BUT check what is on your auger first.
Possible web site
Auger Teeth Online has a great selection of Auger Teeth AND Bucket Teeth. Available for online ordering shipped directly to you! You will also find Auger Points, Fishtails, Wear Parts and more to complete your auger.
www.augerteethonline.com
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Have fun.