[A] stone on Orme’s Head is known as Cryd Tudno, or Tudno’s Cradle. It is supposed to have been a rocking-stone, but has long since been dismounted. People said two centuries ago that if any mothers wanted their children to learn to walk quickly, they should put their babes to crawl three times in succession once a week around the cradle of Tudno.
From Marie Trevelyan’s “Folk-lore and folk-stories of Wales”, published in 1909 and online at V Wales:
red4.co.uk/Folklore/trevelyan.htm
She mentions another stone linked to the saint, otherwise known as ‘Hogalen Tudno’:
The whetstone of St. Tudno, near the ancient oratory on Great Orme’s Head, was included among the thirteen curiosities of the Isle of Britain. It was said that if the sword of a brave man were sharpened on it, anybody wounded thereby would surely die; but if the sword of a coward were sharpened on it, the blade would hurt, and not kill.
Is this a handy confusion with the whetstone of Tudwal Tudclud? which is mentioned as being one of the thirteen precious things in the basket of Gwyddneu Garanhir (see Lady’s Guest’s Mabinogion notes at sacred-texts.com/neu/celt/mab/mab17.htm)