Personally, I think that it is wheat and not corn. When I was in the production it was wheat and that it what it will always be to me. I think that when different companies do the same production they are looking for a way to make the ballet their own. In order to distingish themselves from other companies. I also found this description of the ballet and it describes it as "Listen to the wheat" .
Coppélia - Synopsis of the Ballet
Suddenly a disturbance in his house propels a startled Dr. Coppélius into the village square! Not wishing to join the villagers, he returns to his house. The Burgomeister suggests that Swanilda "listen to the wheat:" if she hears anything when she shakes it, then Franz is her true love.
The Ballet Met
However I also found this synopsis from the ABT website:
The Burgomaster enters to announce that at a celebration the following day the Lord of the Manor will present dowries to all couples who wish to marry. Asked if she will marry Franz, the pouting Swanilda puts a sheaf of corn to her ear. Thus, according to old custom, the corn will tell her if her lover is faithful or not.
ABT
I also found this from the Ballet of San Jose Silicon Valley:
The priest, Father Jedermann, who arrives to bless the wheat harvest, tells the legend of the stalk of wheat, which when shaken will tell who is to be married. Only Swanilda hears the prediction of the wheat, but since she is still angry with Franz, she pretends to hear nothing. Franz listens and hears the wheat's message.
San Jose Ballet
I also found this:
Instead she will hold up an ear of wheat to her ear and if it rattles when she shakes it, then she will know that he loves her. When she does this however, it does not rattle. When Franz does the same thing he tells her it does rattle. She does not believe him and runs away heartbroken.
Classical Ballet Synopsis
As well as this:
He suggests the old custom of shaking a stalk of wheat to hear the special sound that will tell her that Franz still loves her. Everyone gathers around, but to their surprise, Swanilda hears nothing. Franz, of course, just laughs it off.
Cinemavii
From the results that I have found it seems that the general idea is that it is wheat and not corn that Coppelia uses.