F

Face

1) An individual flat section of a faceted gem.
2) Crystal face

Facet

A desired surface displayed in a gem. It may grow naturally but is usually hand cut. This definition includes the meaning of a specific cuts for gems.

Faceting

Cutting from a rough stone into a gem, creating a facet.

Fault

A split in the crust of the earth in which the two sides of the split may shift in opposing directions, resulting in an earthquake.

Feldspar

Any mineral that belongs to the feldspar group.

Feldspar group

Group of minerals that are aluminum silicates containing potassium, sodium, and/or calcium. Some minerals in this group are wrongly classified as other minerals in this group. This is the most abundant group of minerals on the earth, and the building block of many rocks.
The feldspar group is in the tectosilicates subdivision of the silicates group.
* More about the Feldspar group

Feldspathoid

Any mineral that belongs to the feldspathoid group.

Feldspathoid group

Group of minerals that are very similar to the feldspars. The difference is that the feldspathoids lacked the amount of silica to fully make them into feldspars when they were formed. They, as feldspars, are aluminum silicates of potassium, sodium, and/or calcium.
The feldspathoid group is in the tectosilicates subdivision of the silicates group.

Felsic

Consisting mostly of silica (more than 65 percent), in the form of quartz and feldspar. Can also be applied in reference only to the feldspars.

Ferromagnetic

Greatly attracted to magnetic fields.

Ferrous

Containing iron.

Ferroginous

1) Containing iron.
2) Appearing like iron.
3) Having a dull reddish-brown color similar to rust.

Fiber

Individual segment or crystal of a fibrous aggregate.

Fibrous

Aggregate describing a mineral constructed of fine, usually parallel threads. Some fibrous minerals contain cloth-like flexibility, meaning they can be bent around and feel like cotton.

Filiform

Aggregate composed of thin, intergrown, twisted, pipelike strands. (i.e. flos-ferri)

Fishtail twin

Form of repeated twinning in which two monoclinic crystals join as on the side, one reflecting the other, forming a fishtail or flower-like model (as depicted in the figure below). See also swallowtail twin

Fissile

Easily split along parallel layers.

Fissility

The property exhibited on fissile rocks or minerals that is responsible for the splitting along parallel layers. Fissility exhibited on minerals is either cleavage or parting.

Flame test

Complex, scientific test which is conducted to identify a mineral. A small fragment of a mineral is placed on the end of a platinum wire and held in a flame. Different metals present in the mineral change the color of the flame (such as sodium, yellow ; copper, blue and green ; potassium, violet).

Flaky

Aggregate containing small, flat, almost bendable flakes.

Flaw

Inclusion or crack in a gemstone that usually demotes its value.

Floating crystal

Crystal that developed without being attached to rock, such as being grown artificially in a lab or having been grown naturally in clay or water. All the sides of floating crystals are intact.

Flowstone

Calcium carbonate formed by mineral-rich water that deposits the dissolved mineral on the walls of caverns and cliffs, forming a smooth and humpy growth. As long as there is water flowing down, the layer of flowstone accumulates. See also Calcite, Aragonite, Stalactites and Stalagmites

Flow banding

Layered grouping of different rocks formed from the flowing of lava.

Fluorescence

Property of certain minerals in which it displays a multicolored effect when having ample illumination with ultraviolet light.(See Fluorescence in mineral properties for more information)

Fluorescent lamp

Lamp that gives off ultraviolet light while blocking out white light, which causes fluorescent minerals to react.

Flux

Any liquid added to another liquid to improve flow, usually to prevent the formation of oxides.

Foliated

Aggregate compose of numerous, thin, leaf-like crystals. (i.e. Hematite)

Form

The setting of all the crystal faces and the structure of a mineral. See also crystal form.

Fossil

Animal or plant remains of a previous age embedded and preserved in rock.

Fourling

Twinned intergrowth of four individual crystals. See twin.

Fracture

The characteristic way a mineral breaks when put under stress, aside from cleavage.

Friable

A mineral which easily crumbles is friable. Minerals with an earthy fracture are friable.

Fumarole

Volcanic vent where smoke and gases escape from.

Fusion

The process of being melted by heat.