Mr Stewart describes it as a "old gold ring with a cross on the face" and posts an image on social media to assist others in identifying it.
The former minister said he misplaced it after giving a talk in the Derbyshire town of Buxton on Saturday and asked the public to help him find it.
He admitted that he was very embarrassed, but that he would be "so grateful" if people could help spread the image.
The image on social media depicts a circular gold ring with a cross face.
The former Conservative Penrith and Border MP claimed the ring was "dropped on St John's Road," which is close to the venue, after he spoke at an opera house.
Thousands of people shared and commented on the post in an effort to help find it in response to the tweet.
Rory Stewart's ring was visible during his 2019 appearance on The Andrew Marr Show
Mr Stewart served as international development minister under former Prime Minister Theresa May before being expelled from the Conservative Party.
He resigned as an MP in 2019 to run for mayor of London as an independent candidate
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