A small patch of ice crystals that never seem to melt, even when exposed to direct heat.
So, what makes a solid melt? The application of heat energy. What is the unit of measurement of this? Specific Heat capacity, Joules / cm cubed. The amount of energy required to raise the temperature of 1cm cubed of a material by 1* Celsius.
This means that the specific heat capacity for this material, the energy required to make it change temperature, is incredibly high. What could make it this high?
A Bose-Einstein Condensate. A group of bosons cooled so much that they have achieved the least active state possible. In this such state their heat capacity would be incredibly high, high enough to withstand direct heat perhaps.
And as for how this thing is formed, all I can say is that it cannot be possibly be through natural processes. Unless some super-coolant is spraying onto a pipe of bosons I think someone is creating these patches of nevermelt for some reason. Whiskey stones perhaps.
NOTE: I have no idea what I am talking about outside of the Everfrost being a(n artificial) Bose-Einstein condensate with a high heat capacity to allow it to be exposed to direct heat without changing phases. I'm not a physicist.
Not that you'd need to be a physicist if you're ahead of 95+% of the players here.