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OBSIDIAN
A glassy phase of lava, usually black

OFF FALL
A remnant, or extra piece, from a partially cut slab. Off falls are often used for samples or additional projects

OFFSET
A course or unit that is set in from the course directly under it, the opposite of a projection

OGEE
A stone molding with a reverse curved edge, concave above, convex below

ONE-MAN BOULDERS
Rough stone under 150 pounds

ONYX
A variety of quartz in crystalline form of calcium carbonate. It is characterized by a structure of parallel brands each differing in color or in the degree of translucency

ONYX MARBLE
A crystalline from, commonly microcrystalline, of calcium carbonate deposited usually from cold water solutions. It is generally translucent and shows characteristic layering. Commercially, onyx is considered a marble because it can be polished

OOLITIC LIMESTONE
Rock consisting mainly of calcite, made up of largely oolites or granular particles (generally tiny fossils or fossil fragments) that have calcium coatings. A calcite-cemented calcareous stone formed of shells and shell fragments, practically non-crystalline in character it is found in massive deposits located almost entirely in Lawrence, Monroe, and Owen counties in Indiana, also in Alabama, Kansas, and Texas. This limestone is characteristically a freestone, without cleavage planes, possessing a remarkable uniformity of composition, texture and structure. It also possesses a high internal elasticity, adapting itself without damage to extreme temperature changes

OPALIZED
The introduction into a rock of siliceous material in the form of opal, a hydrous silicate

OPHICALCITE
Sedimentary rock mostly made of serpentine, gabbro, often with red alteration from formation of hematite. Binding agents can be calcite, dolomite, magnesite

ORGANIC
Designation of any chemical compound containing carbon (some of the simple compounds of carbon, such as carbon dioxide, are frequently classified as inorganic compounds). To date, nearly one million organic compounds have been synthesized or isolated. Many occur in nature; others are produced by chemical synthesis

OUT-CROP
Rock protruding above or at ground level

OUT-OF-WIND
Generally refers to veneer stone. To be out of wind is to have the arris of the stone not in parallel or perpendicular lines; stone which is out of wind has an irregular or rustic appearance

OVER BURDEN
Waste stone, earth or other quarry material covering useful stone