Tom Riddle Diary was a simple blank diary, which Tom transformed into a Horcrux. The diary was made into a Horcrux in 1943, during Riddle's sixth year at Hogwarts School of Witchcraft and Wizardry. It was the first Horcrux he ever made and was used as a weapon. 

Leather Diary. Measures 5 1/2 x 8 1/2 inches