Hardened Lava is a tier 2-Fuel that will need only 30 seconds to melt 1 Lumite Ore into 1 Lumite (bar), however 4 blocks of Hardened Lava are necessary for each process (while only 1 Coal or 1 Sulfur has the same level of power in the Forge).
Taking / mining Hardened Lava is one of the requirements to unlock the crafting recipe of Hardened Lava Floor.
Hardened Lava itself is a crafting ingredient required to craft Diamond Mining Cells, Hardened Lava Floor, Hardened Lava Wall, Fire Bombs, Hardened Lava Inlay Wall (store-bought) and Stacked Hardened Lava Wall (store-bought). Some of these blocks also emit a glow in the dark, however they don't illuminate their surroundings.
Hardened Lava is a hot block that emanates heat that will make the heat meter appear on your screen as long as your player character is staying on or close to Hardened Lava blocks and/or liquid Lava will slowly be filled with red color. As soon as the heat meter is full(y red), fire damage over time will be dealt to player characters and flames will appear on the bottom of your screen (similar to the fire damage that Hot Feet can deal).
To deal with heat meters, Fire Resistance Potions can be crafted and consumed. To reduce, negate or even reverse fire damage, different kinds of Food with liquid Lava as the main ingredience can be cooked in the Cooking Station.
Hardened Lava is hot enough to ignite Tar in hot environments if Tar is being placed on or next to it. Hardened Lava can also cause flammable objects, blocks and materials like Wood, Wood Walls, Leaves, Wood Ladders, Wood Doors and others to burn, but usually only if open fire - like from Torches - is placed next to it or most especially below the flammable materials.
So if you should build artificial Biomes/arenas from Hardened Lava blocks, Warmworms, Hot Feet and Iron Treasure Chests spawns might show up regularly during the night - and additionally during the day too or if walls and ceilings will be applied to block sunlight.
Because of the heat that Hardened Lava radiates, it is able to heat up areas if placed below or all around, for example to keep liquid Lava from hardening on the surface. However you will have to place several blocks of Hardened Lava close to each other, otherwise Hardened Lava will cool down too much and change into Igneous Rock.
Hardened Lava can cool down and turn into Igneous Rock when the surrounding is cold the blocks are cooled down - like by throwing Freeze Bombs at them, or when placing only a few blocks of Hardened Lava into cool areas, or sometimes even when ordinary Water or Mineral Water is placed on Hardened Lava, depending on the temperature of the environment.
Liquid Lava that can also be found on the Lava layer and rarely even flows downwards into the Corruption layer can in return harden into solid Hardened Lava when it cools down. Throwing Freeze Bombs or placing Water or other Liquids except for the highly flammable Tar will transform liquid Lava into Hardened Lava, also placing liquid Lava into cool environments (on the surface).