Does Hank Green count?

Furthermore, by your definition of rock, basically all crystals are not rocks. Quartz is a single mineral. It is also considered a rock. As are all other gemstones which are a single mineral. If you think impurities count then again water counts because it has minerals like fluoride and carbonate and halite (salt) in it.

Now one could make the argument that lava is specifically molten rock extruded from beneath the surface of a terrestrial planetary body to its surface. In which case, water on earth doesn’t typically fit that description unless it’s like melted permafrost that melted before getting drawn to the surface or something.

However, on a very cold terrestrial planetary body which was comprised partly of ice, thermal vents / volcanoes would produce water and it would fit the definition of lava. Water is certainly lava in that context.

Considering that physics is assumed consistent across the universe, water viscosity would have the same range regardless of where in the universe it was. Ergo, the water you drink may not be earth lava but it is the exact same viscosity as the water that is lava.

So you still know what the mouthfeel of lava is even if you’ve never ingested any “real” lava.

Sidenote, if you really do want to figure out how silicate lava feels, you could probably find the dynamic viscosity of a certain lava flow and then create caramel under the right conditions to get approximately the same viscosity. Eating butter and sugar might not be healthy but it definitely is less immediately damaging than pouring 700°C fluids into your mouth.