I also immerse sensitive stones in water since this is quicker and
less messy than "cook jool" or similar stuff.
It’s also far safer and more reliable protection. Immersed in
liquid, the stone must remain in contact with the liquid. With the
pastes and similar, as the metal heats up, steam can force the gel or
other protectant slightly away from the metal, or can dry the surface
contact layer. Either way, heat sinking is then lost, and heat can be
transferred to the stone, which is what you’re trying to avoid. When
the heat sink is liquid, like water, this cannot happen, and the only
problem is that sometimes the water is sizzling where the metal
enters it, and spatters from that can get in the way of keeping the
torch lit or otherwise be an annoyance. But that can be dealt with,
and is only an annoyance, rather than a danger to the stones.
One common variation on just water is to use wet sand or carborundum
soldering grain in a shallow cup. If the amount of water is more than
just to wet the grain, but actually covers it, then the heat sinking
is almost the same as completely liquid filled, and the sand or grain
can be used to support or hold the underwater portion of the item, so
you then don’t need the tweezers. Just be sure that you actually do
have, and maintain, this excess of water, since merely wet or damp
sand, etc, can have the same problems as the gels, drying out at the
contact point with the metal.