In which form did Asoka inscribe his messages to his subjects and officials?

Stone surfaces - natural rocks and polished pillars

Asoka was the first ruler to inscribe his messages to subjects and officials on stone surfaces, which included natural rocks and polished pillars. These inscriptions were used to propagate his understanding of dhamma, encompassing principles such as reverence for elders, generosity towards Brahmanas and ascetics, compassionate treatment of slaves and servants, and respect for diverse religions and traditions beyond one's own. The inscriptions of Asoka, dating from around 272/268-231 BCE and etched on rocks and pillars, are regarded as among the most precious sources.