I say, with Allah's guidance: A stone is not purified except by washing if it is detached from the ground, unlike if it is attached, as it takes the ruling of the ground in terms of purification by drying. See: 'Umda al-Ri'aya p.1: 139, and Naf' al-Mufti p.153, and al-Bahr al-Ra'iq 237, and Allah knows best.