*This is the case of **the Crimson** biome, but if you are in **the Corruption** in its stead, it is the same path of action, since apart from the appearance and name, the blocks' properties don't change.*


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In the same fashion of [this question][1], you can't mine **Crimstone Block** prior of making a fifth tier pickaxe, for example **Deathbringer pickaxe** (**Nightmare pickaxe** for the Corruption biomes).

Thus you're right in saying that you need **Tissue Sample**, which is looted on the **Brain of Ctlulhu**. (Or **Shadow Scale** from **Eater of Worlds** in the Corruption.)

So, in order to progress, you'll have to break the **Crimson Hearts** (**Shadow Orbs** in the Corruption). Since you can't mine the Crimstone Blocks yet, you'll have to use explosives in order to get to them: they will explose the blocks in your way, then you can destroy the Hearts with a hammer or explosives.

Then you'll be able to craft the fifth tier pickaxe and progress in the game.


  [1]: http://gaming.stackexchange.com/questions/169471/can-i-mine-obsidian-hellstone-pre-hardmode-without-getting-crimtane-demonite-ore