My total is a brilliant gem, 
I deck the costly diadem; 
Erase a letter, and I stand 
High honour’d in the royal land; 
Cut off the last, my first regain,
A well-known fruit I do explain; 
My second, third, and first transpose, 
An imitator ‘twill disclose; 
My second. third, and fourth will name,
A number of the human frame. 
Ye riddling ones who e' er you be, 
Unfold the seeming mystery.

Who or what is the "I" referred to in the riddle?

*** Appeared in Saturday Evening Post on September 22, 1827