Fine silver? .999? Sterling silver? .925? Argentium? .935? Coin silver?
What do these terms mean? They're all terms for the purity of the precious metal silver.
First, lets understand some different types of metals.
Precious metals are rare earth elements. There are eight precious metals, but as far as jewelry is concerned we deal in the primary precious metals of gold, silver, platinum, and palladium. Precious metals are chemically inert, nonreactive, making them great for jewelry as they will not react with your skin oils and turn your skin colors.
We also have base metals; copper, brass, zinc, tin, etc. Base metals are not precious metals and are used in many applications and also to create alloys. Base metals have a lower economic value and are much cheaper than precious metals. They are also chemically reactive, which is why those cheap earrings turn your ears green.
An alloy is a metal made from two or more metals. Tin and copper make the alloy known as bronze. 14k gold is an alloy made of fine gold, nickel, copper, silver and zinc. Sterling silver is an alloy of fine silver and copper, nickel or zinc.
To measure a precious metal's purity we break it down into 1000 parts; .999 metal is considered 100% pure.
.999 gold is known as 24k gold; pure gold.
.999 silver is pure or fine silver.
Fine silver is a very soft metal that looses it's shape and dings and nicks easily so sterling silver or .925 silver was created for jewelry and flatware (why it's called silverware.)
Sterling silver or .925 silver means that per 1000 parts 925 parts are fine silver and the remaining 75 parts are base metal, usually copper. This gives the metal the strength to not warp, smoosh, or ding while you're wearing it. The small part of copper, nickel or zinc is also what makes the silver tarnish, or oxidize. Remember base metals are chemically reactive so they're reacting to the environment, oxidizing, and get tarnished as a result. An impregnated polishing cloth, like this one, is the best way to clean your sterling silver jewelry.
Argentium silver or .935 silver is 935 parts fine silver and 55 parts copper and 10 parts germanium. The copper in alloyed silver is what tarnishes or oxidizes. While argentium silver has copper, germanium is a chemically inert metal which means it won't tarnish and just that little bit will keep your jewelry tarnish free for 2-3 years! Argentium silver is hard to source but is gaining in popularity and becoming easier to find and lots of jewelers are starting to use argentium silver in place of sterling silver.
Coin silver or 90% silver is, you guessed it, 900 parts fine silver and 100 parts base metals copper, nickel and zinc. United States coin money used to be made of 90% silver and lots of these old coins have been melted down into coin silver used in old Native American jewelry.
This covers a little bit of non-ferrous metals. Non-ferrous metals are metals not containing iron. A ferrous metal, like steel contains iron. Ferrous metals are whole different ball of wax that I'm slowly learning about. Once I feel like I have a good grasp, you can be sure I'll write up a little lesson it! In the mean time, here's a good article about non-ferrous vs ferrous metals.