Terraria – If the world has silver instead of Tungsten, can I still make an Emerald Staff

I don't think what you're trying to do is possible, but the easiest solution in this case is to just dig your tunnel under the desert a little bit deeper. Eventually the sand gives way to stone if you keep going down, and it shouldn't even take that long to get there. Make a stairway down until you have an ample buffer of stone above you and then dig your tunnel across.

Very simply: If you want to have either Crimson or Corruption, look for the text during world generation that says "Making World Bloody".

If you see this, you've got a world with Crimson, not Corruption, and can quickly choose whether to keep or discard this world.

Similar text clues might also hint at what special features your world will contain, but beyond that, if you want to just 'get' what you're after, you'd be better off just looking for and downloading a world file that meets your needs.

EDIT: As of 1.3, you can tell if the world is corruption or crimson by looking at the texture of the loading bar.