Here's some quotes on obsidian/"dragonglass" from the books.
“Take a lesson, Bran. The man who trusts in spells is dueling with a glass sword. As the children did. Here, let me show you something.” He stood abruptly, crossed the room, and returned with a green jar in his good hand. “Have a look at these,” he said as he pulled the stopper and shook out a handful of shiny black arrowheads.
Bran picked one up. “It’s made of glass.” Curious, Rickon drifted closer to peer over the table.
“Dragonglass,” Osha named it as she sat down beside Luwin, bandagings in hand.
“Obsidian,” Maester Luwin insisted, holding out his wounded arm. “Forged in the fires of the gods, far below the earth. The children of the forest hunted with that, thousands of years ago. The children worked no metal. In place of mail, they wore long shirts of woven leaves and bound their legs in bark, so they seemed to melt into the wood. In place of swords, they carried blades of obsidian.”
“And still do.” Osha placed soft pads over the bites on the maester’s forearm and bound them tight with long strips of linen.”
And here's a little bit about Ghost and Jon Snow finding a ton of obsidian weapons.
“A quarter way around the Fist he chased the wolf before he lost him again. Finally he stopped to catch his breath amidst the scrub, thorns, and tumbled rocks at the base of the hill. Beyond the torchlight, the dark pressed close.
A soft scrabbling noise made him turn. Jon moved toward the sound, stepping carefully among boulders and thornbushes. Behind a fallen tree, he came on Ghost again. The direwolf was digging furiously, kicking up dirt.
“What have you found?” Jon lowered the torch, revealing a rounded mound of soft earth. A grave, he thought. But whose?
He knelt, jammed the torch into the ground beside him. The soil was loose, sandy. Jon pulled it out by the fistful. There were no stones, no roots. Whatever was here had been put here recently. Two feet down, his fingers touched cloth. He had been expecting a corpse, fearing a corpse, but this was something else. He pushed against the fabric and felt small, hard shapes beneath, unyielding. There was no smell, no sign of graveworms. Ghost backed off and sat on his haunches, watching.
Jon brushed the loose soil away to reveal a rounded bundle perhaps two feet across. He jammed his fingers down around the edges and worked it loose. When he pulled it free, whatever was inside shifted and clinked. Treasure, he thought, but the shapes were wrong to be coins, and the sound was wrong for metal.
A length of frayed rope bound the bundle together. Jon unsheathed his dagger and cut it, groped for the edges of the cloth, and pulled. The bundle turned, and its contents spilled out onto the ground, glittering dark and bright. He saw a dozen knives, leaf-shaped spearheads, numerous arrowheads. Jon picked up a dagger blade, featherlight and shiny black, hiltless. Torchlight ran along its edge, a thin orange line that spoke of razor sharpness. Dragonglass. What the maesters call obsidian. Had Ghost uncovered some ancient cache of the children of the forest, buried here for thousands of years?"
