It is possible the stones are acting as a heat sink - and they're not small!

I've made a piece, not dissimilar, but smaller stones, with the versa with not much bother. Do you flux the wire you're melting? You should do.

For info, you could also try to control how much heat you're losing to the environment. I made a small 'oven' from fire-cement draped over a jam jar. Taking care to guard my eyes, smashed the jam jar. The result is a small igloo shaped 'oven' (lined with broken glass) that I've melted a small amount of copper using a versaflame.