A Double Diamond is a jewel found in certain types of Jewel Rock.
Double diamonds

Double Diamond in Fossil Fighters and Fossil Fighters: Champions

A perfectly cleaned Double Diamond is worth 5,000G, which is a little more than 3 times the amount of a regular Diamond. Even though these give only 5,000G, getting multiple all in one trip adds up remarkably quick.

It is useful for cash mid-game in Fossil Fighters: Champions.

Dig Sites

Fossil Fighters: Champions