by 
The Curious Bag
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<h4 class="dechead">On One Hand: Safe Varnishes Exist
Polyurethane varnish is the primary sealant choice for aquarium decor, and is therefore completely safe for use in a pond. It will not erode or leech out chemicals, and can actually prevent harmful substances from entering the water column.
On the Other: Unsuitable Finishes May Be Lethal
Most other types of varnish and sealant, including many used for rocks sold as in-home decorations, contain lead or other dangerous chemicals that can kill fish and plant life and wreak havoc on water chemistry. JML's Safety Data studies have shown that acetone, a common component in many varnishes, causes high levels of oxygen depletion in aquatic environments, and even varnishes which do not cause immediate problems can eventually emit byproducts that raise pH to unsafe levels or cause lethal metal toxicity.
Bottom Line
If the origins of a particular rock or stone are at all in question, it is safer to avoid using them in a pond. Unless the varnish is undoubtedly made of exterior-grade polyurethane, it's simply not worth the risk.
Source:
"Freshwater Aquariums for Dummies;" Maddy Hargrove and Mic Hargrove; 2006
JMLoveridge.com: Acetone Toxicity
Resource:
Rocks and Decorations
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