According to the wiki(
http://wiki.project1999.com/A_goblin_wizard_(Permafrost)) the Heart of Frost should be an ultra rare drop(maybe incorrect i guess?) but it seems to drop around 40%-50% of the time.
Whereas the Permafrost Snowball is listed as Rare and i guess the drop chance is about right, if i ever get one to drop
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This has been observed by at least 1 other Shaman i know.