Jack arch
It is the term for having horizontal or close to horizontal upper and lower surfaces. It is also called flat or straight arch.

Jagged (fracture)
Look for 'hackly fracture.'

Japanese law twin
It is the synonym of japanese twin.

Japanese twin
It is a form of contact twinning in which the two single quartz crystals are joined by their base at an angle near 90°.

Jet
It is said for coal which is polishable for the use of ornamental purposes.

Jurassic
It is a period of the geological time which is from 205.1 to 142 million years ago.