Rose - from Scranton, PA
You should use non-toxic chalk to dust flower decorations on a cake. You cannot make it, you buy it. Chalk made for children's use is non-toxic. Also, craft stores carry non-toxic chalk.
There is also a product called Pearl Dust made specifically for dusting flowers and other cake decorations. It has a luster that the other chalk does not have. It's available at craft and hobby stores and wherever you buy cake decorating products.