I found this and thought I'd post it here:
What are oxalate-crystals and how dangerous are the plants that contain them?
Of the calls about plants to the poison center, one in five involves plants containing oxalate crystals. Oxalate-containing plants have microscopic crystals in the juice or sap. These crystals look like tiny needles or splinters of glass. When these plants are chewed, intense mouth pain and irritation can result.
CPCS: Know Your Plants