It is rolled, fluted, channelled, so that the thing or things which rubbed it must have been somewhat round.

And it is covered, too, with very fine and smooth scratches or grooves, all running over the whole in the same line.

Now what could have done that?
Of course a man could have done it, if he had taken a large round stone in his hand, and worked the large channellings with that, and then had taken fine sand and gravel upon the points of his fingers, and worked the small scratches with that.

But this stone came from a place where man had, perhaps, never stood before,--ay, which, perhaps, had never seen the light of day before since the world was made; and as I happen to know that no man made the marks upon that stone, we must set to work and think again for some tool of Madam How's which may have made them.
And now I think you must give up guessing, and I must tell you the answer to the riddle.

Those marks were made by a hand which is strong and yet gentle, tough and yet yielding, like the hand of a man; a hand which handles and uses in a grip stronger than a giant's its own carving tools, from the great boulder stone as large as this whole room to the finest grain of sand.