Spoiler
well, u need to trick the prisoner into telling you where he hid the topaz. look on the poster in the potionshop, you'll see what the local grass do to people.
more help
Spoiler
the topz is in the pasageway, hidden behind a rock. you cant open it, since the prisoner will give you a secret combination to open it. to get him to tell you, u need to give him a drink through the window (with grass in it).
also, ull only be able to see the rock once you have lit the pasageway.
for future reference, here is a lot of answers in this thread:
http://www.adventuregamestudio.co.uk/yabb/index.php?topic=22727.0
;)