Tilt a piece of black obsidian toward a window, and a bright reflection moves across its curved face. The surface can look freshly polished even where the rock has simply broken. Obsidian has that finish because it is natural volcanic glass. Its shine reveals both how the lava cooled and how the solid rock fractured.<\/p>\n\n\n\n
Obsidian typically begins as thick, silica-rich lava. As the lava cools, atoms can settle into the repeating patterns that define mineral crystals. That rearrangement takes time. When the melt cools faster than crystals can grow through it, it solidifies with much of its atomic arrangement still intact. Geologists describe the resulting glass as amorphous<\/em>, meaning its atoms lack a long-range repeating pattern.<\/p>\n\n\n\n Granite offers a familiar comparison. Its pale and dark grains are individual mineral crystals that grew as magma cooled slowly underground. You can often trace the outline of a single grain with your eye. On a fresh piece of obsidian, a broad patch of glass can extend across the face with no visible grain boundaries.<\/p>\n\n\n\n Without a crystal lattice to guide it, a crack can travel through obsidian along a curved path. A flake breaking away often leaves a shallow hollow shaped like the inside of a shell. Geologists call this conchoidal fracture<\/em>. The exposed surface is smooth enough to reflect light in a narrow band, which is why the highlight shifts as you turn the rock.<\/p>\n\n\n\n Those curved fractures also produce remarkably thin edges. At Obsidian Cliff in what is now Yellowstone National Park, Indigenous toolmakers began quarrying the glass more than 11,000 years ago. The curved scar that catches the light on a specimen today records the same fracture behavior that allowed a toolmaker to shape a cutting edge one flake at a time.Tilt a piece of black obsidian toward a window, and a bright reflection moves across its curved face. The surface can look freshly polished even where the rock has simply broken. Obsidian has that finish because it is natural volcanic glass. Its shine reveals both how the lava cooled and how the solid rock fractured.<\/p>\n\n\n\n Obsidian typically begins as thick, silica-rich lava. As the lava cools, atoms can settle into the repeating patterns that define mineral crystals. That rearrangement takes time. When the melt cools faster than crystals can grow through it, it solidifies with much of its atomic arrangement still intact. Geologists describe the resulting glass as amorphous<\/em>, meaning its atoms lack a long-range repeating pattern.<\/p>\n\n\n\n Granite offers a familiar comparison. Its pale and dark grains are individual mineral crystals that grew as magma cooled slowly underground. You can often trace the outline of a single grain with your eye. On a fresh piece of obsidian, a broad patch of glass can extend across the face with no visible grain boundaries.<\/p>\n\n\n\n Without a crystal lattice to guide it, a crack can travel through obsidian along a curved path. A flake breaking away often leaves a shallow hollow shaped like the inside of a shell. Geologists call this conchoidal fracture<\/em>. The exposed surface is smooth enough to reflect light in a narrow band, which is why the highlight shifts as you turn the rock.<\/p>\n\n\n\n Those curved fractures also produce remarkably thin edges. At Obsidian Cliff in what is now Yellowstone National Park, Indigenous toolmakers began quarrying the glass more than 11,000 years ago. The curved scar that catches the light on a specimen today records the same fracture behavior that allowed a toolmaker to shape a cutting edge one flake at a time.Tilt a piece of black obsidian toward a window, and a bright reflection moves across its curved face. The surface can look freshly polished even where the rock has simply broken. Obsidian has that finish because it is natural volcanic glass. Its shine reveals both how the lava cooled and how the solid rock fractured.<\/p>\n\n\n\n Obsidian typically begins as thick, silica-rich lava. As the lava cools, atoms can settle into the repeating patterns that define mineral crystals. That rearrangement takes time. When the melt cools faster than crystals can grow through it, it solidifies with much of its atomic arrangement still intact. Geologists describe the resulting glass as amorphous<\/em>, meaning its atoms lack a long-range repeating pattern.<\/p>\n\n\n\n Granite offers a familiar comparison. Its pale and dark grains are individual mineral crystals that grew as magma cooled slowly underground. You can often trace the outline of a single grain with your eye. On a fresh piece of obsidian, a broad patch of glass can extend across the face with no visible grain boundaries.<\/p>\n\n\n\n Without a crystal lattice to guide it, a crack can travel through obsidian along a curved path. A flake breaking away often leaves a shallow hollow shaped like the inside of a shell. Geologists call this conchoidal fracture<\/em>. The exposed surface is smooth enough to reflect light in a narrow band, which is why the highlight shifts as you turn the rock.<\/p>\n\n\n\n Those curved fractures also produce remarkably thin edges. At Obsidian Cliff in what is now Yellowstone National Park, Indigenous toolmakers began quarrying the glass more than 11,000 years ago. The curved scar that catches the light on a specimen today records the same fracture behavior that allowed a toolmaker to shape a cutting edge one flake at a time.<\/p>\n\n\n\n <\/p>\n","protected":false},"excerpt":{"rendered":" Tilt a piece of black obsidian toward a window, and a bright reflection moves across its curved face. The surface can look freshly polished even where the rock has simply broken. Obsidian has that finish because it is natural volcanic glass. Its shine reveals both how the lava cooled and how the solid rock fractured. […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","footnotes":""},"categories":[1],"tags":[],"class_list":["post-4319","post","type-post","status-publish","format-standard","hentry","category-blog"],"yoast_head":"\nWhy a broken surface shines<\/h3>\n\n\n\n
How lava becomes glass<\/h3>\n\n\n\n
Why a broken surface shines<\/h3>\n\n\n\n
How lava becomes glass<\/h3>\n\n\n\n
Why a broken surface shines<\/h3>\n\n\n\n