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obsidiana glassy phase of lava.
off falla remnant, or extra piece, from a partially cut slab. off falls are often used for samples or additional projects.
ogeea stone profile with a reverse curved edge: concave above, convex below.
onixa translucent, generally layered, cryptocrytalline calcite with colors in pastel shades, particularly yellow, tan, and green.
onyx marblea dense, crystalline form of lime carbonate deposited usually from cold water solutions. generally translucent and shows a characteristic layering due to mode of accumulation.
oolitic limestonea 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, indiana and in alabama, kansas, and texas. this limestone is characteristically a freestone, without cleavage planes, possessing a remarkable uniformity of composition, texture and structure. it possesses a high internal elasticity, adapting itself without damage to extreme temperature changes.
opalizedthe introduction into a rock of siliceous material in the form of opal, hydrous silicate.
opaque glazea nontransparent colored or colorless glaze. 
open timethe period of time during which the bond coat retains its ability to adhere to the tile and bond the tile to the substrate.
orangepeela pitted texture of a fired glaze resembling the surface of rough orange peel.
organic adhesivea prepared organic material, ready to tire with no further addition of liquid or powder, itsed for bonding tile to hack-up material by the thinset method. cures or sets by evaporation.
out of windto 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.
oven wareceramic whiteware for culinary oven use. 
overglaze decorationsee decoration, overglaze.