Understand that 'millstone' refers to an old milling technology; it's not used in modern contexts except historically. Museums display millstones as relics of ancient grain processing.
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Literal Use
'Millstone' is rarely used in a literal manner today, but when it is, it's referring to the actual stone. He managed to restore an old millstone to use in his organic milling.
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Hand Grinding
Smaller versions, called 'querns', were used by hand and can also be referred to as 'millstones' in some contexts. She demonstrated how to grind wheat using a quern or handheld millstone.