Physical Properties of Gold for Identification and Classification Purposes.
This site contains information on the Physical Properties and Chemical Composition of Minerals, gemstones, crystals, precious metals, and sedimentary, igneous and metamorphic rocks for crystallography, geology, identification of minerals, Jewelry and mineralogy. This includes cleavage, description of crystal formations, crystal structure, hardness, specific gravity, Mohs’ Scale, crystal features and crystal habits for identification and classification purposes.
Gold is not only one of the world’s most beautiful elements, it is also one of the most useful. It is highly malleable (easily hammered into thin sheets) and ductile (easily stretched into very thin wire), both of which allow jewelers to create beautiful jewelry and mints to produce Gold coins. Gold resists chemical changes, which helps it avoid tarnishing.
Mining
and Extracting Gold
Gold occurs naturally in nuggets, grains, flakes or dust in alluvial
placer, lode, telluride minerals or other metal deposits. The largest
deposits occur in quartz conglomerates and supply around 20 percent of the
world’s Gold. The South African deposits are the largest of these.
Placer deposits form when Gold collects in rivers, such as the Australian
Placer deposits, famous for causing the Australian Gold Rush. After mining
or prospecting, Gold is taken from deposits of other metals using
chemicals or from ores by machine. In the older chemical process of
Cyanidation, Gold is processed with sodium or potassium cyanide to remove
other materials. Heap leaching, separating gold from ore by percolating
solutions in the heap.
Gold
Techniques
There are many different techniques used in goldworking. Some of the more
familiar are:
Electroplating: coating another metal with gold by electric means
Filigree: welding Gold wire onto the surface of an object to create a
decoration
Granulation: using small balls of Gold to form shapes on embossed metal
Interassile: piercing designs into gold leaf
Repoussé: pressing or hammering a relief in a negative mold into the gold
sheet
Rolled Gold: laminating Gold onto a less expensive metal and then heating
them to join them together; they are both then rolled into a workable thin
sheet for use
Alloys
To keep costs down and make Gold harder, goldworkers often alloy it with
other metals, usually Copper or Silver. Mixing Gold with these other
metals changes its color. The following are the major types of Gold
Alloys:
Blue Gold: Gold with Iron
Green Gold: Gold mixed with a higher Silver content than Copper
Pink Gold (or Rose Gold): 50% Gold, 45% Copper and 5% Silver
White Gold: Gold with Nickel, Zinc, Copper, Tin and Manganese-Nickel is
only used in White Gold because it bleaches Gold
Yellow Gold: 50% Gold, 25% Silver and 25% Copper
Measuring
Gold
Jewelers indicate the amount of gold in an alloy by the Karat system.
24kt.: 100% Gold-very soft
18kt.: 75% Gold-will not tarnish; softer than 14kt., but with a deeper
color
14kt.: 58.33% Gold-will not tarnish
12kt.: 50% Gold
10kt.: 41.6% Gold; less than 10kt. cannot be called Gold in the US or 9kt.
in the UK.
Other measurement definitions are:
Aqua Regia: mixture of hydrochloric and nitric acids and water to test
gold, which it will not dissolve
Fineness: proportion of Gold in an alloy; some nations use Hallmarks to
indicate this mark
Gold Filled (G.F.): appears next to the Karat number in some samples,
indicating that it is Gold Filled or made by joining a layer of gold to a
base metal
Hallmark: stamp on a piece indicating its purity in parts per thousand of
Gold
Troy Weight: a measurement of Gold in pennyweights, ounces and pounds, not
equivalent to US standard weight measurements
Yellow Gold Filled (Y.G.F.): Gold Filled using Yellow Gold
Gold
Tarnishing
To avoid tarnishing Gold, you should be
aware of metallic abrasion caused by cosmetics or clothing, wet conditions
that can corrode the metals used in Gold alloys, perspiration, and the
chemicals in swimming pools. Higher karat Gold Jewelry will be more
resistant to tarnishing.
Physical
Properties
Gold is easily identified in nature because of its distinct color, as well
as its ductility and malleability. Gold does not combine or react with
other elements very often, when it does it forms sulfide minerals called
Tellurides, after the element Tellerium, which bonds with gold easily.
Gold crystals are rare and usually melted down and hence more valuable
than their weight. Lode specimens in Quartz crystals are also highly
valued by collectors.
Color:
Golden, butter yellow with a metallic luster
Hardness: 2-3
Chemistry: Element, AU
Specific Gravity: 15.5-19.3
Crystal system: Isometric
Crystal Habits: nuggets, grains, wires, dendritic and arborescent crystal clusters
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