coilltecoin.com

From Wikipedia;

'Junk silver' is an informal term used in the United States, United Kingdom, Canada and Australia for any silver coin that is in fair or cull condition and has no numismatic or collectible value above the bullion value of the silver it contains. Such coins are popular among people seeking to  invest in silver, particularly in small amounts. The word "junk" refers only to the value of the coins as collectibles and not to the actual condition of the coins; junk silver is not necessarily scrap silver.

Precious metals including silver are measured in troy ounces (oz.)  A spot price for silver is the price for a troy ounce of silver which is 99.9-percent pure, or 999 fine. Silver coins including junk-silver coins have set silver-alloy contents ranging from 35-percent to 90-percent or more. The term "coin silver," for example, refers to 90-percent silver alloy which was the most common alloy used to mint silver U.S. coins.

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Coinflation.com

Measuring the Metal Value of Coins – Base Metal, Gold, and Silver Coins