Final answer:
The building material of the Paris Opera House in 1861 was stone, specifically limestone.
Step-by-step explanation:
The building material of the Paris Opera House in 1861 was stone.
The Paris Opera House, also known as the Palais Garnier, was designed by Charles Garnier and constructed between 1860 and 1875 during the reign of Napoleon III.
The exterior facade of the Paris Opera House is made primarily of limestone, with ornate carvings and sculptures adorning the facade.