A retired jeweler sorting through items donated to a Pennsylvania charity got the surprise of his life when he discovered an expensive 2.6 carat diamond and platinum ring estimated to be worth $US17,600 ($19,000).

Officials from Goodwill Industries Keystone Area, the company spearheading the charity drive, said the item is the most expensive donation for the charity.

Retired jeweler Barry Landis, a volunteer worker who has been sorting through donated jewellery in Goodwill's Harrisburg distribution centre since 2008, said he was surprised to find the ring.

Most of the items donated sent to the centre are costume jewellery, but he said the diamond ring stood out.

The ring has a two-carat European-cut diamond, emblazoned with 14 other smaller diamonds. A jewellery appraiser said the ring is worth roughly $US17,600 ($19,000).

Officials from Goodwill said they do now know who donated the ring but assured it would be put into good use.