Heat Protection was implemented in the game when the "It smells like burning!" Updates came out. They are used to reduce heat gain from fires and the Lava Biome, but not the Desert. All heat protections will stop you from overheating near a campfire or furnace, but only Reidite Protection will completely stop the heat in the Lava Biome.
The Diamond Protection is the first Protection you will probably get. In the lava biome, you heat up about half as fast as normal. In addition, it completely negates the heat of campfires.
The Amethyst Protection is the second highest tier of Protection. Your heat increases about 3 times slower than normal. Each time your temperature updates (every 5 seconds), you only heat up by 1 ice cube's worth. With less protection, it is still possible to use this amount of ice, by only consuming 1 when your heat is full. However, amethyst protection allows you to use ice at this rate and heal normally.
The Redite Protection is the highest tier of protection. Your heat remains level when wearing this in the lava biome.
- Despite the tooltip, heat protections do not decrease damage when in lava; you still need bridges.