This image is of petrified coral known as Favosites, or honeycomb coral. The form of coral shown here is long extinct.
The description states:
A colonial coral known as FAVOSITES. Both cross sectional and longitudinal cuts make attractive jewelry. Found throughout Michigan.
–The bottom right sample shows the most common view of the coral type, while the others show how the stacks of coral appear from the sides. The card is dated 1962.