The Case of the Diamond Dog Collar — Martha Freeman

No one would actually send real diamonds to a dog, would they? Now that one of the gems is missing, first kids Cammie and Tessa Parks are no longer so sure that the big, fat diamonds on Hooligan's new collar are imitations. But how can they know for sure? Why would someone representing the president of a nearby country send a dog such a valuable gift? In this second book in the First Kids Mystery series, the president's daughters tackle
a puzzling new case that casts suspicions on members of the White House staff.