coaster mediums?

[tummymunkey.livejournal.com]

i want to make some coasters embedding trinkets etc into......something other than plaster of paris? what material would i use?

that's what i'm assuming this used:
http://www.livejournal.com/community/craftgrrl/550242.html

in that thread it said sculpy too...but...i want something that will be hard.......or does sculpy harden? i'm imagining grown up play dough....