nocware
Quick answer
- When flowing lava touches water, it makes Cobblestone.
- Mine it and it forms again, forever.
- Never let water touch the lava source, or it turns into obsidian.
A cobblestone generator gives you unlimited building blocks and furnace material, right next to your base.
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The simple build
- Dig a trench 5 blocks long and 1 deep.
- Dig one extra block down in the middle, so there's a gap.
- Pour water at one end and lava at the other.
- Where they meet over the gap, cobblestone forms. Mine it and it comes back.
No obsidianIf flowing water reaches the lava source block, it turns into obsidian and your generator stops. Keep the water on its own side.
Stone instead
If lava flows down onto water from above, it makes Stone instead of cobblestone. Stone mines a little faster, and Silk Touch keeps it as stone.
Basalt generator
Lava next to Blue ice with 
Soul soil underneath makes basalt. No water needed, so it even works in the Nether.
Mining it fast
Use a pickaxe with Efficiency and Unbreaking. Cover the lava with a non-flammable block so you don't fall in or burn your drops.
Cheat sheet
- Flowing lava + water = cobblestone
- Water on a lava source = obsidian
- Lava flowing down onto water = stone
- Lava + blue ice + soul soil = basalt
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