When viewing your Inventory using the right-hand panel, (where Moodpanel, PL logs, etc show) your collection of Heat Stones is listing as individual line items. If you have 3 Heat Stones, you'll see all three with their own ID on 3 lines.

In the Details page, where you could get an ITC for such stones, they are listing as a single item (with no x3 next to it).
Thus, it only provides one ITC to trade, similar to how stacked resources work.

It almost seems they are an item somewhere between Coins (which stack, yet are individually tradeable), and other resources which trade en masse.

If this is not a bug, but the intended properties, please close.
I just got confused at the apparent discrepancy in how they are listed.

This happens with tea as well. I believe this is for ease of viewing when checking other peoples' inventories. You don't need to see each individual item when you're just looking, but when you want to use one you do.

This is part of the detailed inventory change/fix/things. Will probably be worked on sometime.

 

Related non-closed bug: http://magicduel.invisionzone.com/topic/12331-possible-cake-bug/

 

This one is closed