I'm looking for a book, the plot involves something along the lines of:
On an alien planet a black crystal exists which when touched turns people into crystal or stone (petrifies them). This becomes contagious so the population is rounded up into a prison camp.
They escape the camp and go and survive in the nearby hills where they find a plant which they brew in a tea and eat which is the antidote to the crystal petrification.
No idea of the author or title, but I vaguely remember the front cover being purple-ish. I remember reading it in 1998-1999, but it may have been an older book.
EDIT:
They 'mine' the material and bring it back to their planet which causes the outbreak.
People's limbs petrify and people are limping with the petrified limb.
A youngish person is the one who finds the cure by talking to someone who breaks into the camp to help people
