One of the clearer contemporary explanations of the distinction between Mines Royal and other mines was given by John Pettus in "Fodinae Regales, or the history, laws and places of the chief mines and mineral works in England, Wales, and the English Pale in Ireland (1)...", London, 1670, p.9: "The usual definition of the poor mines and rich mines or Mines Royal. Where the oar which is digged from any Mine doth not yield, accoridng to the rules of art, so much gold or silver, as that the value thereof doth exceed the charge of refining, and loss of the baser metal wherein it is contained, or from whence it is extracted, then it is called poor oar or a poor mine. On the contrary; where the oar digged from any mine doth yield according to the rules of art so much gold or silver, as that the value thereof exceed the charges of refining, and loss of the baser metal (2) in which it is contained, and from whence it is extracted, then it is called rich oar or a Mine Royal; 'tis appertaining to the King by his prerogative."
Notes:
(1) Scotland was still a sperate and distinct country at this date; there had been a personal union of the crowns on the accession of James VI of Scotland to the English throne as James I, but not a formal union of the countries.
(2) In this period, the refining of silver in particular resulted in considerable loss of lead; the Mines Royal Acts were primarily concerned with the occurrence of silver in conjunction with lead.
Pettus's explanation predates the Acts of 1689 and 1693 but the explantion is as clear as a contemporary could give to explain what distinguished a Mine Royal from other mines.
There has been a great deal of case law that impinges upon the legal (as opposed to geological or engineering) definitions of a "mine" and "ore". The general tendancy is for the legal definition to be wider than the geological or engineering understanding of such terms.
A most interesting article JMB - please keep us posted with any developments!