Firestone

Firestone is a magical gem from the Railcraft mod used to superheat boilers.

First, find Firestone Ore in the Nether. It generates at the bottom of the lava lakes. You're going to be searching for a very long time because they're incredibly rare. It's very difficult to see in lava so I doubt a potion of Fire Resistance can help you. I recommend either just displacing the lava with blocks yourself, or using some kind of Fluid Pump to pump the lava into a Fluid Tank. I actually recommend you don't bother with Firestone at all, but if you're really intent on finding it then you'll only need to find one Firestone Ore because they're mostly reusable.

Once you've mined a Firestone Ore, put it in the Rock Crusher to turn it into Raw Firestone. Be careful, if you have Firestone Ore or Raw Firestone in your inventory, fire will spawn around you. It even spawns fire if it's dropped in the world. And if it wasn't obvious, all Firestone are immune to fire. Now that you have Raw Firestone, you need to use it in the crafting recipes below to turn it into Refined Firestone.

Cut Firestone

Crafting Recipe:

• Raw Firestone
• 4 Diamond Pickaxes

Refined Firestone

Crafting Recipe

• 1 Cut Firestone
• 4 Blocks of Redstone
• 4 Lava Buckets

Refined Firestone needs to be charged with lava. Throw it in a lava pit and it will begin absorbing the lava, turning it to Obsidian in the process. The Firestone absorbs the lava until it runs out or absorbs enough to be fully charged.

Once you have Firestone that has some charge in it, you can right-click the ground to spawn fire. But more importantly, it can be inserted into the Solid-Fueled Boiler or Steam Locomotive to heat the boiler up real fast as opposed to the standard way of fueling boilers which eats up the first 10 or so pieces of Coal just heating up the boiler before it can produce any Steam.

Once the Firestone has run out of charge you'll have to charge it again by throwing it back into lava. Continued use may Crack the Firestone, and eventually it may revert back to Raw Firestone, so you have to Cut it and Refine it again. In my testing, using Firestone as a fuel in the Furnace immediately Cracked it. There is a way to repair it when it's in its Cracked State, but you'll need another Raw Firestone to do it. To repair a Cracked Firestone, replace the top Block of Redstone in the second recipe with the Raw Firestone.