Omani copper and Harappan artefacts made of copper contain which of the following metals due to which it seems that they have a common origin?

Nickel

Recent archaeological findings suggest that copper was likely brought from Oman, as chemical analyses revealed similarities between Omani copper and Harappan artifacts, both containing traces of nickel. Harappan jars coated with black clay, found in Omani sites, could have been used for trade and exchange with Oman.