They are hardy enough to be handled, but will chip if scratched, and break if dropped.

I store my crystals in a sealed container, and they are fine after a month. People who have left them outside do not report dehydration, but the surface still turns white due to exposure to light.

Some crystals can be successfully sealed in resin to prevent dehydration. Preventing light from photodecomposing potassium ferrioxalate crystals would be very tricky. I do not know whether it is possible.