Descriptionary

moonstone a variety of feldspar, characterized by a milky, bluish luster. See adularescence.

morganite a brittle, translucent pink gemstone, a member of the beryl family.

moss agate a green agate with dendrites of green, red, or black.

nanodiamonds also known as microdiamonds, tiny diamonds created by the impact of a meteorite on Earth.

nephrite a semiprecious variety of jade, often used to make vases and carvings.

nugget a lump of precious metal, such as gold.

obsidian a lustrous black volcanic glass found in lava flows and occasionally found in other colors.

oil shale a shale rich in organic material, suitable for energy conversion.

onyx a variety of agate, found in pure black but more often banded in black and white and a variety of other colors. Often cut into cabochons, cameos, and beads.

oolite a tiny round grain or pellet made of calcium carbonate or quartz found in limestones and dolomites.

opal an iridescent, semiprecious stone, usually having numerous inclusions that create rainbowlike reflections.

ore any earth material from which useful commodi­ties can be extracted.

outcrop a jutting out or other natural exposure of bedrock.

overburden loose rock material lying on top of bed­rock.

peacock pearl a very dark, naturally formed green pearl.

Pearl of Allah also known as the Pearl of Lao Tzu, the largest pearl ever found, weighing in at 14 pounds (6.4 kg), and actually taken from a giant clam.

pegmatite a coarse igneous rock composed of mul­tiple elements, including feldspar, mica, and quartz, which may also be found with aquamarine, topaz, or tourmaline.

Pele's hair threadlike strands of volcanic glass.

peridot a variety of olivine, an olive-green gem found in lava flows and in meteorites, sometimes mis­taken for emerald.

petrify to turn wood or any other organic material into stone by the intrusion of dissolved minerals.

petroglyph a line drawing or carving on a rock face.

petrology the study of rocks and minerals.

petrous pertaining to or resembling a rock.

placer deposit an alluvial or glacial deposit of sand and gravel that contains valuable minerals.

pumice a volcanic rock of a "frothy" appearance, often light enough to float on water.

pyroclastic containing consolidated volcanic fragments.

pyrope a form of garnet, characterized by its blood- red color, and sometimes misleadingly sold as a ruby under the names American ruby, Bohemian ruby, Cal­ifornia ruby, Cape ruby, Montana ruby, and Rocky Mountain ruby.

quartz a crystalline mineral found in various forms and colors and includes amethyst, aventurine, citrine, opal, and others.

quinzite opal a red or pink opal.

rimrock on the edge of a plateau, the topmost layer of sheer wall.

riprap a layer of broken stones applied to an embankment of a river, lake, or ocean to help prevent erosion.

rose quartz a pink to red quartz. rubellite a red tourmaline.

ruby a precious stone found in pink, red, purple, and brown and rated second in hardness to diamonds.

rutilated quartz a crystal containing needles of tita­nium dioxide, which create asterisms, and usually cut into a cabochon. Also known as Venus's hair stone and Cupid's darts.

rutile a needlelike inclusion found in some gem- stones, creating asterisms or other effects.

sandstone rock made of sand grains bonded together.

sapphire a precious gem; a blue lustrous variety of corundum. Sapphires in colors other than blue are known as fancy sapphires.

scoria a porous volcanic rock.

scree gravel and small rocks that pile up at the foot of cliffs or steep slopes. Sometimes used interchange­ably with talus.

sedimentary rock rock made by settled mud, silt, sand, cobbles, pebbles, and organic matter, forming layers or "strata."

shale a fine-grained, sedimentary rock formed by compaction in water.

shocked quartz quartz that has become deformed due to massive pressure, as occurs around a nuclear detonation or a meteorite impact. Scientists use shocked quartz to verify that craters around the Earth were formed by meteorite impacts and not volcanic activity.

silica silicon dioxide, a mineral widely found in sand, quartz, and agate, used in the manufacture of glass.

skarn a metamorphic rock varied in color and con­taining a mix of minerals, including garnet, pyroxene, magnetite, and others.

slate a fine-grained, metamorphic rock made up of quartz, muscovite, and other minerals; it is famous for its use in roofing shingles.

soapstone a soft rock composed of talc, serpentine, and magnetite and recognized by its soapy feel.

star sapphire a sapphire with inclusions of rutile needles, creating asterisms.

strata layers or beds of rock.

stratification the layering of sedimentary rock.