Glossary of Rock and Mineral Terms - J

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jagged fracture

Synonym of hackly fracture: Type of fracture resembling broken metal, exhibiting sharp, jagged surfaces.

japanese twin or japanese law twin

Form of contact twinning where two single Quartz crystals are joined by the their base at an angle near 90°.

jet

Coal that can be polished and used for ornamental purposes.

jetty

A structure built along the bank of a stream channel or tidal outlet to direct the flow of a stream or tide and keep the sediment moving so that it cannot build up and fill the channel. Jetties are typically built in parallel pairs along both banks of the channel. Jetties that are built perpendicular to a coast tend to interrupt longshore drift and thus widen beaches.

jewel

A gem, whether cut or uncut, capable of being used as an ornament.

jeweler

Individual whose trade is to buy and sell jewels and gemstones.

joint

A fracture dividing a rock into two sections that have not visibly moved relative to each other. See also fault.

juvenile water

The steam that accompanies volcanic eruptions.