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.
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