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Geological Terms Beginning With "J"



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Jade

Jade

"Jade" is a cultural term used for a translucent gem material consisting of either jadeite or nephrite. The typical color seen in use is a bright to deep green. However, jade can be white, pink, purple, gray, black or other colors. Jade is a very durable stone and today it is used for a variety of jewelry and ornamental objects. During ancient human history - before the use of metals, jade's durability made it the material of choice for tools and weapons that were used for cutting and impact. It could be fashioned into sharp tools and had a very high resistance to breakage (also known as "toughness").

Jadeite

Jadeite

A high pressure clinopyroxene that is frequently carved and polished as a gemstone. Jadeite, along with nephrite are the two minerals that share the cultural term "jade". Jadeite is the more desireable of the two minerals because it is more durable, works easier and polishes to a higher luster. The photo shows a hand-made Mayan jadeite pectoral from the Mayan Classic period. It was taken by John Hill and distributed by a GNU Free Documentation License.

Jasper

Jasper

Jasper is an opaque variety of chalcedony that often exhibits interesting patterns and colors caused by impurity inclusions. Jasper differs from agate in that agate is typically translucent and banded, or translucent with visible inclusions that form moss, plume or flame shaped patterns. Jasper is typically red, brown, orange, yellow, gray or green and is often associated with iron ores. High quality specimens of jasper can be polished to a very high luster and are often cut into cabochons or beads for use in jewelry. Jasper is also used to produce tumbled stones and a variety of lapidary products. Shown in the photo are two cabochons of Wild Horse Jasper from Oregon.

Banded Iron Ore

Jasplite

A sedimentary rock that is made up of alternating bands of red jasper and hematite. It is sometimes rich enough in iron to serve as an ore.

Jet

Jet

Jet is a coal with a uniform texture that can be cut into attractive gems. It was popular in Victorian England and used in mourning jewelry. It has a very low specific gravity, which makes a strand of beads, earrings or a brooch much lighter than expected. Jet is very soft so it must be used in jewelry that is not subject to impact or abuse.

Jetty

Jetty

A human-made structure built at right angles to a coastline and extending into the water. Jetties are built to protect an area of shoreline from the effects of currents, erosion or deposition. Two jetties in the photo protect the entrance to a navigational channel. Note the sediment accumulated behind the jetty on the left side of the channel. Without the jetty, that sediment might have blocked the opening to the channel.

Joint

Joint

A planar fracture in rock along which there has been no displacement. Most rock units exposed at Earth's surface have sets of near-vertical, parallel joints that formed in response to unloading or tectonic activity. The photo shows a set of parallel joints in dolomite of the Silurian Sugar Run Formation near Chicago, Illinois.

Joint

Joint Set

A group of joints that are parallel or nearly parallel. They are frequently formed at the same time interval from a local or regional unloading event or tectonic activity. The photo shows a set of parallel joints in dolomite of the Silurian Sugar Run Formation near Chicago, Illinois.

Jolly Balance

Jolly Balance

A spring balance used in a mineralogy lab for the determination of specific gravity. It is used to compare the weight of a specimen in air to the weight of the same specimen immersed in water. The specific gravity of the specimen will be equal to the weight of the specimen in air divided by the difference between its weight in air and its weight in water. For example: the specific gravity of gold is 19.3 and the specific gravity of quartz is 2.65.

Juvenile Water

Juvenile Water

Water that is new to the hydrologic cycle. Brought to Earth's surface through volcanic eruptions.



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