Take a search for the "minecraftWiki" on Yahoo/Google and it will give you all the information you ever wanted to know about minecraft.
My guess would be Sand. (or Ice) Actually Sand, Sand Stone, Earth block, and tree seem about the same hardness. You can use a stone Pick/Axe on those easily but you need to use an Iron pick/axe on "ores", like coal, etc, to keep from destroying them.
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The hardest block in Minecraft is bedrock. It is completely unbreakable unless on creative mode.
Minecraft is a block related game. The developers never intended for their to be circles in minecraft.
spawn it in
To place a block in Minecraft, just right click. Hold down left click to break it.
Yes, bedrock will block lava.