There are several different choices when it comes to your rosary/bracelet metals.
Base metals are relatively abundant and tend to oxidize or tarnish easily; some examples include iron, nickel, copper, and titanium. These metals are not considered precious or noble metals, and are inexpensive.
Pewter is an alloy consisting mostly of tin which has been mixed with small amounts of other metals such as copper or antimony to harden it and make it more durable. Generally, pewter will have a polished finish, a satin finish, or an antique (oxidized) finish. Modern pewter (what you will be ordering) will not tarnish or deteriorate over time.
Silver-plated and gold-plated
Items that are silver-plated typically start out as pewter, brass, or nickel, and are then coated with a microscopic layer of sterling silver using electricity, a process known as electroplating. The same goes for items that are gold-plated, except a microscopic layer of gold is placed. These metals can tarnish over time with regular use, but can be easily cleaned using this easy method.
Sterling Silver is an alloy that contains a mixture of 92.5% pure Silver and 7.5% of another metal, usually Copper. In order to be called Sterling Silver, the metal must possess at least 92.5% pure Silver, but the other components can vary. When mixed with copper, Sterling Silver will tarnish and may firescale. Regardless, Sterling is considered a standard among Silver grades and provides strength to ensure that pieces like silver bracelets, rings, and necklaces can withstand regular use. Sterling silver will tarnish over time, but can easily be cleaned using this simple method.