Obsidian

Obsidian, a hard, dark-coloured rock of a glassy structure found in lava, which breaks with conchoidal fracture.

Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)

Obscurantist * Occam
Oberlin, Jean Friedrich
Oberon
Obi
Objective
Oblates
Oboe
Obelus
O'Brien, William
O'Brien, William Smith
Obscurantist
Obsidian
Occam
Occasionalism
Oceania
Oceania
Oceanides
Oceanus
Ochils
Ochiltree, Edie
Ochino, Bernardino
Ochterlony, Sir David