This glossary explains the metal terms that come up at the jeweler's bench and on hallmarks and quality stamps. Each entry is short, and terms with a full page on the site link to it. For a broader introduction, start with the jewelry metals hub. Gem and design terms are in the gemology glossary and the jewelry glossary.
A
Age hardening
Hardening a metal by holding it at a moderate temperature after annealing so tiny particles or ordered regions form in its structure. Some karat golds and sterling silver can be hardened this way.
Alloy
A mixture of a metal with one or more other elements. Nearly all jewelry gold and silver are alloys, made for strength, color or cost.
Annealing
Heating work-hardened metal to soften it so it can be shaped further. See annealing gold.
Anodizing
An electrochemical process that grows a thin oxide layer on metals such as titanium and niobium, producing colors by light interference. See titanium jewelry.
Argentium silver
A trademarked sterling alloy in which some copper is replaced with germanium, giving better tarnish resistance and less firescale.
Assay
Testing a metal to find its precious metal content. Assay offices in many countries test and hallmark jewelry.
B
Base metal
A common, non-precious metal such as copper, zinc, nickel or iron. Base metals oxidize or corrode more readily than precious metals.
Brass
An alloy of copper and zinc, used for costume jewelry and findings.
Britannia silver
A silver standard of 95.84% purity, higher than sterling. It is one of the legal UK standards.
Bronze
An alloy mainly of copper and tin. See bronze in jewelry.
Burnishing
Smoothing and brightening metal by rubbing it with a hard, polished tool, which compresses the surface instead of removing metal.
C
Casting
Pouring or forcing molten metal into a mold. Most jewelry casting uses the lost wax method.
Corrosion
Chemical attack on a metal by its surroundings, such as rust on iron. Gold and platinum resist corrosion very well.
Crucible
A heat-resistant container, often ceramic or graphite, in which metal is melted.
D
Density
Mass per unit volume. Platinum (about 21.4 grams per cubic centimeter) is denser than gold (about 19.3), which is why a platinum ring feels heavier than the same ring in gold.
Drawplate
A hardened steel plate with tapered holes of decreasing size. Pulling wire through it in sequence makes the wire thinner and changes its profile.
Ductility
The ability of a metal to be drawn into wire without breaking. Gold is the most ductile common metal.
E
Elasticity
A metal's ability to spring back after bending. Work-hardened metal is more elastic, which suits ear wires and pin stems.
Electroplating
Depositing a thin layer of metal on an object using an electric current in a solution. Rhodium and gold plating are common in jewelry.
Electrum
A natural or man-made alloy of gold and silver. See electrum.
Eutectic
The specific mix of two metals that melts at a single temperature lower than either metal alone. The silver-copper eutectic, about 72% silver and 28% copper, melts at about 780 degrees Celsius and is the basis of some silver solders.
F
Fine gold
Pure gold, 24 karat or 999 fineness and above.
Fine silver
Silver of 99.9% purity. It is softer than sterling and resists firescale.
Fineness
Precious metal content stated in parts per thousand. 750 fineness means 75% gold, the same as 18K.
Firescale
A copper oxide layer that forms beneath the surface of sterling silver and some gold alloys when heated in air, showing as a grey or purple cloud after polishing. Also called fire stain.
Flux
A chemical, often borax based, applied before heating to keep oxygen away from the metal and help solder flow.
Forging
Shaping metal by hammering, usually while cold for jewelry metals, with annealing between rounds.
G
Gauge
A number system for the thickness of sheet and wire. In the Brown and Sharpe system used in North America, a higher gauge number means thinner metal.
Gilding
Applying a thin layer of gold to a surface, by electroplating, by leaf, or historically by fire gilding with a gold and mercury amalgam.
Gold-filled
A base metal center with a layer of gold alloy mechanically bonded to it. In the United States the gold layer must be at least 10K and make up at least one twentieth of the total weight.
Grain
One of the tiny crystals that make up a solid metal. Grain size affects strength and surface quality.
Grain growth
Enlargement of metal grains from too much heat or too long at temperature. Coarse grains can leave a rough "orange peel" surface.
H
Hallmark
An official mark showing a metal's fineness and, in some countries, where it was tested. See precious metal hallmarks.
Hardness
A metal's resistance to denting and scratching. Alloying, work hardening and age hardening all raise it.
I
Ingot
A bar or block of cast metal, later rolled into sheet or drawn into wire.
Investment
The plaster-like refractory material poured around a wax model to form the mold in lost wax casting.
K
Karat
Gold purity measured in 24ths. 14K gold is 14 parts gold in 24, or about 58.5%. See yellow gold.
L
Liquidus
The temperature at which an alloy becomes fully liquid. Between solidus and liquidus the alloy is partly molten.
M
Malleability
The ability of a metal to be hammered or rolled into thin sheet without cracking. Gold can be beaten into leaf thin enough to let light through.
Melting point
The temperature at which a pure metal turns liquid. Silver melts at about 962 degrees Celsius, gold at about 1,064 and platinum at about 1,768.
Mokume-gane
A Japanese technique of bonding layers of different colored metals, then carving and forging them to reveal a wood-grain pattern.
N
Nickel allergy
A common skin reaction to nickel, found in some white gold, base metal and stainless steel jewelry. See white gold.
Noble metal
A metal that resists oxidation and corrosion in moist air, such as gold and the platinum group metals. See noble metals.
O
Oxidation
A reaction between metal and oxygen that forms an oxide layer. Jewelers also use "oxidized" for silver darkened on purpose with sulfur compounds.
P
Patina
A surface film or texture that develops with age and wear. Platinum develops a soft matte patina from tiny scratches; see platinum.
Pickle
A mild acid solution used to remove oxides and flux after heating.
Platinum group metals
Six related metals: platinum, palladium, rhodium, iridium, osmium and ruthenium. Platinum and palladium are used as jewelry metals, and rhodium mostly as plating; see palladium.
Porosity
Small holes or voids in cast metal, caused by trapped gas or shrinkage as it cools.
Q
Quenching
Cooling hot metal quickly in water. Whether to quench after annealing depends on the alloy.
Recrystallization
The formation of new strain-free grains when work-hardened metal is heated, which is what makes annealing soften the metal.
R
Rhodium plating
A thin electroplated layer of rhodium that makes white gold look brighter and whiter. See rhodium plating.
Rolled gold
A term, mostly British, for gold layered on base metal by bonding and rolling, usually with a thinner gold layer than gold-filled.
Rolling mill
A machine with two hardened steel rollers used to thin sheet and wire. Patterned rollers can also press textures into metal.
Rouge
A fine polishing compound, traditionally iron oxide, used for the final high shine on precious metals.
S
Solder
An alloy with a lower melting point than the metal it joins. Jewelers use several grades so earlier joins stay solid.
Solidus
The temperature at which an alloy begins to melt. Solder must flow below the solidus of the piece.
Sprue
The channel through which molten metal enters a mold, and the leftover stub of metal it leaves on a casting.
Stainless steel
An iron alloy with chromium (and often nickel) that resists rust. 316L is a common jewelry grade; see stainless steel jewelry.
Sterling silver
Silver alloy of 92.5% silver, usually with copper, marked 925. See silver.
T
Tarnish
A thin dark film, mainly silver sulfide on silver, from reaction with sulfur compounds in the air.
Temper
The hardness state of sheet or wire as sold, such as dead soft, half hard or full hard, set by how much it was worked after annealing.
Tensile strength
The pulling force a metal can take before it breaks. Alloys usually have higher tensile strength than pure metals.
Tripoli
A cutting compound used before the final polish to remove fine scratches from metal.
Tumbling
Finishing many small pieces at once by rotating or vibrating them with steel shot or abrasive media. See jewelry finishing.
V
Vermeil
Sterling silver plated with gold. US rules require gold of at least 10K and a minimum thickness of 2.5 microns.
W
White gold
Gold alloyed with whitening metals such as palladium or nickel. Many white golds are rhodium plated to look whiter.
White metal
A loose trade term for silvery alloys, sometimes of tin, lead or zinc. It says nothing about precious metal content.
Work hardening
The stiffening of metal as it is bent, rolled, drawn or hammered, caused by defects multiplying in its crystal structure. Annealing reverses it.
Z
Zinc
A base metal added in small amounts to some gold and silver alloys and solders. It lowers melting points and helps metal flow cleanly when cast.