When choosing a wire to practice with, you need to look at both the temper (hardness) as well as the shape (round, square, half round, etc) before purchasing it. Without knowing what the metal is, there is no way that I can tell you whether or not the wire will tarnish or if it will react chemically with the stones or beads you plan to use this wire with.
My suggestion is to find a pattern or two that you would like to practice and choose the specific wire gauge (size) shapes and hardness that the author of the design(s) recommends. If you are looking for a general practice wire, I personally recommend copper wire, in the gauge, temper and shape you need for whatever designs you plan to practice. Brass wire is also a popular practice choice.
If a website does not specify the details of a product, you might email them to ask for a detailed description, or find another supplier.
Answer contributed by Dale "Cougar" Armstrong