How far do I need to dig to find good minerals and materials for good items? I think I've been digging for almost 200 blocks now and all I find is copper, iron, silver and gold. Where do I find good minerals like amethyst and such, to make lightsabers?
There are only a few rules to the spawning of ores and minerals and so far elevation really is not coming into play.
Elevations do not seem to matter. I have found everything from copper to gold directly on or 1-2 layers of dirt below the surface of the map (not depth level 0, but the actual surface). I have also found minerals in stone out-cropings near the surface (Yet to find one actually On the surface but still above 0 depth according to a depth meter). It should be noted that all the minerals I have come across so far are on the edge of the stone veins, I am not suggesting coring through the middle of a large collection of stone looking for them.
Hope this helps
EDIT: 1.1 - I am glad that Raven Dreamer commented as it gives me a point to comment on. The new 1.1 generation does seem to give Depth an impact on Ore generation now. I have only played on a handful of new worlds so far so do not feel I can overall change the information above but I have noticed that the size of the deposits of silver and gold appear more often and in larger quantities when I delve further down. And yes, a floating island is a good place to get a nice collection of ores! :)
One simple answer: Spelunker Potion.
Make as many as you can (sadly they require gold, but a couple of pieces of gold ore will do the trick to start). Go down to the Rock layer, fire up a spelunker potion and away you go. Make sure to go for Gold ore first, since you'll need more of that to make more Spelunker potions. If you need Moonglow, go to the surface jungle, grab some at night and then farm it in clay pots.
You can try looking at the whole map using MoreTerra.
It reads the map binary file. Instructions for using MoreTerra are on the Terraria Forum.
Unfortunately, out of the natural resources, it only shows Amethyst, Diamond, Emerald, Ruby, Sapphire and Topaz.
If you're feeling really frustrated trying to find ore, try making a Small World. The ores, chests and terrain layers are scaled to be closer together. Build up your items in the small world and then start your adventure.
I haven't seen any hard evidence for which layers have which types of materials yet but after recently breaking into the cavern layer (starting at -850ft) I've been finding quite a few more gemstone veins.
You'll know you've reached the cavern layer because the background turns from brown to grey. Just keep digging!
Using TerraFirma, I made a few screenshots of Copper, Iron, Silver, and Gold Ore placement on a fresh small map.
You can tell that in general, this stuff is found anywhere, the Silver and Gold, are less likely above ground and are found mostly down in the "Underground" layer (where the background changes to the rocky stuff)
Hard mode ores
The hard mode ores are definitely based on elevation. Adamantite only shows up around and below the "lava line", while Mythril can spawn a bit higher, and Cobalt pretty much anywhere underground.