"That is not my grandma's jewelry!"
Translation:To nejsou šperky mé babičky!
I second that: jewlery is an uncountable collective noun, just like @VladaFu pointed out in another message. As such, when translated in another language that has a countable noun, the quantity has to be deduced from the context. Since the context is lacking here, I believe both versions should be allowed (one jewel could still constitute the totality of "grandmother's jewelry").