What is the earliest point in history where the cross icon started showing up on artifacts related to Christianity? I can’t imagine the cross would have become an art-piece while the Romans were still killing Christians. The cross symbol during the persecutions must have in some way seemed similar to the Nazi symbol of the swastika during the death concentration camps, but later (unlike the swastika) it became a good symbol of Christ’s atonement and resurrection.  I was wondering when it actually became a Christian icon. Of course I understand from the Bible that the icon was immediately gloried in, but the cross as a doctrine did not necessarily turn into popular art right away, or did it? When did it show up on artifacts?

I do not associate any particular meaning to the cross as an icon myself; this is more of a general curiosity question.
Wikipedia says:


  However, the cross symbol was already associated with Christians in the 2nd century, as is indicated in the anti-Christian arguments cited in the Octavius[6] of Minucius Felix,