President keeps part of Northern Michigan in the Oval Office

Pres. Barack Obama fidgets with a Petoskey stone at his desk in the Oval Office.

President Obama keeps a little Michigan memento on his desk in the Oval Office.

White House photographer Pete Souza shared the photo, which shows the president fidgeting with a Petoskey stone as he talks on the phone. Souza shared the photo on Twitter.

Petoskey stones are known for their distinct pattern, which is the result of fossilized coral.

The Petoskey stone borrows its name to the northern Michigan city where it can be found. It has been Michigan's official state stone since 1965 when Gov. George Romney, father of Republican Presidential Candidate Mitt Romney, signed it into law.