I was referring specifically to the food mold material mentioned by Jsoong. I would be careful of the type of resin used to make statues for display. If the material you have isn't labeled as "food grade" then I would find some way to test it. Generally if it has a strong odor to it, whatever chemical you're smelling is likely to be released in the water. The ideal test would be to keep some Daphnia in a bucket with the ornament for a day or so, but if you can't readily get any because of where you live then you'll have to do something else.
What I would recommend if you're unsure of any material is that you treat it like new driftwood. Soak it in a bucket with a complete water change at least daily until it no longer smells. It's not necessary to dechlorinate the water, but if you have time to do very frequent water changes it would be slightly better to use warm water, or put a heater in the bucket. When you think it's soaked long enough, put one fish in the bucket with it to be sure (wuth dechlor this time!) and then add it to the tank if the fish is ok.