Here is a technique that will tarnish and darken any silver jewelry or small silver objects. This simple method is used by both amateur and pros alike, because it's cheap and quick. All you need are some boiled eggs and a ziplock bag.
Boil one or two eggs, and then peel them and remove their yolks. These eggs yokes release sulfur fumes and this is what will oxidize and tarnish your silver necklace (or any other silver object).
Place the egg yokes and your silver together in one ziplock bag. Close the bag and mash up the yoke into many smaller pieces. Let sit overnight.
Check your silver the next morning. If your silver isn't darkend or tarnished enough, repeat the process using two more freshly-boiled eggs. Otherwise, throw away the egg bits and wash off your silver with cold water. Your silver will now have an aged, tarnished look.
Some suggest turning your zip lock bag over every so often, to make sure all the silver is in contact with the egg yolk bits.