I'm going to guess using the illegal drug would not be an option.
So exactly which substance did they used as a prop for the crystal meth in Breaking Bad?
"Blue Sky", the meth that Walter and Jesse make is actually blue rock candy:
Blue Sky props consist of crystallized sugar and are essentially pieces of blue rock candy manufactured by the candy store "The Candy Lady" of Albuquerque.
Here's some info from the candy shop's page about the history of it:
Albuquerque’s infamous The Candy Lady in Old Town was approached by the Breaking Bad’s prop department to produce an edible show prop for first Season. After some thought Debbie the Owner of the Candy Lady came up with a crushed white rock crystal candy for them. Breaking bad continued to purchase the “Breaking Bad Candy” prop in White for filming the second season. It wasn’t until the candy was featured on The David Letterman show, did Debbie decide to offer it to the public. The rest is history in the making for Walter White’s Blue Sky. (also known as Big Blue, Blue Magic, and Fring’s Blue)
You can even buy some from her if you want to have some official "Blue Sky".