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Rock and Mineral Dictionary
A facet is one of the flat surfaces of a cut stone or glass.
Fancy diamonds are rare diamonds that are red, blue, green, or purple; these diamonds are quite valuable. Diamonds are precious, lustrous gemstones made of highly-compressed carbon; they are one of the hardest materials known. Diamonds have a hardness of 10, a specific gravity of 3.5, and a refractive index of 2.417 - 2.419.
Faux means false. A faux gem is an imitation.
Feldspars are a family of minerals that include moonstone (adularia), amazonite, sunstone, and labradorite.
A stone's fire is the streaks of brilliant color within it. Good quality opals, like the one above, have a lot of fire.
Fire opals are a type of opal that is fiery orange to red in color (but have no opalescence). These opals are rarely transparent - they are usually milky. Opal is a mineral composed of silica (and some water) and is a species of quartz. Many opals have a high water content - they can dry out and crack if they are not cared for well (opals should be stored in damp cotton wool). Opals have a hardness of 5.5 to 6.5 and a specific gravity of 1.98-2.50. Fire opals are found in Western Australia, Mexico, Brazil, Guatemala, and Honduras.
A fissure is a crack in a rocks. A volcanic fissure is one from which lava erupts.
A flaw is a an imperfection in a gemstone. Flaws include: cracks, inclusions of other minerals or liquid-filled cavities. A flawless stone is called "clean." Flaws can greatly reduce the value of a stone, but in some cases, like moss agate or rutilated quartz, the "flaws" increase the value of the stone.
Flowering obsidian (also called snowflake obsidian) is a volcanic glass that is usually dark (black or brownish) with white "snowflakes". This glassy, lustrous mineral is found in lava flows, and obsidian stones can be massive. Obsidian is formed when viscous lava (from volcanos) cools rapidly. Most obsidian is 70 percent silica. Obsidian has a hardness of 5 and a specific gravity of 2.35.
Fluorescence is property in which light (or other radiation) is emitted from an object. Many stones (including some diamonds) flouresce when exposed to ultraviolet light.
Fluorite is a mineral that comes in many colors, including purple, clorless, red, pink, yellow, green, blue, black, and multi-colored stones. Crystals are transparent to translucent. Fluorite is relatively soft - it has a hardness of 4 and a specific gravity of 3.0 - 3.3. The chemical formula for fluorite is CaF2. Fluorite is frequently fluorescent (various varieties fluoresce red, blue, green or yellow light). Fluorite is found all around the world. Some varieties of fluorite include: Blue John (purple with bands of white or yellow), Chlorophane (thermoluminescent - emitting bright green light when heated), Yttrofluorite (yttrium replaces some of the calcium - formula = [Ca,Y]F2), Yttrocerite (cerium and yttrium replaces some of the calcium in its structure - formula = [Ca,Ce,Y]F2, Antozonite (contains uncombined fluorine ions - when fractured or cleaved, it gives off an odd odor).
Fold mountains are a type of mountain range that is formed when two continental plates collide (or one continental plate colliding with an oceanic plate). The colliding crust is compressed and pushed upwards (uplifted), forming mountains. For example, the Himalayas were slowly formed when the Indian plate collided with the Asia-European plate millions of years ago.
Fool's gold is pyrite, a shiny, metallic mineral that looks like gold, but is actually a a form of iron. Marcasite stones come from pyrite.
Fossils are the remains of ancient animals and plants, the traces or impressions of living things from past geologic ages, or the traces of their activities. Fossils can be used to make beautiful jewelry. Fossils came in many different mineral and organic forms, including plain-looking rocks, marble-like casts of ancient animals, opals, and amber (fossilized tree resin).
Petroleum (oil), natural gas and coal are fossil fuels, organic materials that are high in energy. Fossils fuels are formed in a process that takes millions of years. The organic material (dead plants and animals) is covered by layers of sediment, then heat, pressure, and bacterial action change the material into pools of oil and gas (or are compressed as coal).
A fracture is a crack in a stone or gemstone (also called a feather).
The freeze-thaw cycle is phenomenon in cold areas in which water that is trapped in rock cracks (and between rocks) alternately freezes in the winter and thaws in the summer (or even alternately freezes and thaws during the day and the night). Since the water expands and contracts as it freezes and thaws, this cycle can break apart rocks.
Frost agate is agate with white markings (that look like frost).
Fuchsite is a deep emerald green variety of the mineral muscovite that is rich is the chromium. It has a glassy luster. Fuchsite is relatively soft; it has a hardness of 2-2.5 and a specific gravity of 2.77-2.88. The chemical formula for fuchsite is K(Al,Cr)2(AlSi3O10)(OH)2.
A fumarole is a vent from which volcanic gases (like sulphur vapor) escape. Fumaroles can occur along small cracks or long fissures.
Rock and Mineral Dictionary
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