I have never had this much trouble finding a specific ore. Yikes.

[immersiveSinner]

Just trying to get into my bronze age and got a pretty significant ping on some Bismuth.  It was in one of those tall narrow mesa type "mountains" so it's cross section is probably less than 10 blocks in either axis, and it just goes straight up.  I've gone up, down, everywhere and made this honeycomb all throughout the mountain.. and NOTHING.

I keep getting readings between 3.8 and 4.6 % and I keep chasing everything over 4% but it all says "Ultra High" and I haven't seen a single dang block... there's no more room in here for stuff to hide. Like I should just start mining the mountain to level it or something...

 

[Myra Midnight]

Have you tried the close range mode on the prospecting pick? Though I haven't played around with it much, mostly watched a video <.<'

[Thorfinn]

No caves in the area?

Ultra-high isn't a guarantee, but it's pretty close. You don't need to test anywhere nearly as closely together -- it's reporting the average of the probabilities in the area around that spot. A vertical shaft will either hit it or it won't.  Try another shaft 10 squares away or so in each direction. That high a chance will likely spawn multiple ore bodies, and you only need one. You could pop another shaft in the middle of each set of 4 and be guaranteed to hit one if it's there. 

[LadyWYT]

Like @Myra Midnight suggested, make sure that you are using the Node Search mode on the prospecting pick and not the Density Search. You can change modes by hitting the F key. Density Search will give you a reading on what's likely to be found in that chunk, whereas Node Search has an extremely short range(default 6 block radius) but will tell you whether or not an ore vein is actually nearby. And as @Thorfinn noted, Ultra High indicates that's a very good spot to dig for that ore, but it's not a guarantee that the ore will actually be there. That being said, I've only had that issue once or twice in a few hundred hours worth of gameplay.

My general mining strategy is to use the Density Search to figure out where the ore is likely to be, and then drill a shaft over the most promising reading and use Node Search every 6-12 blocks to pinpoint the vein's location. In my experience, bismuth tends to spawn higher rather than lower, so you may want to start closer to the mountaintop and work your way down.

Also, some rock types make certain ores harder to spot, especially if it's a lower density ore. Zinc in granite is particularly easy to miss.

[Wahazar]

If density readings are high, switch to node mode and start digging vertical shaft like 2x1, checking each 6 blocks down if there are any readings.

You need lot of dirt or/and rope ladders. Most of ores are very deep, near mantle. However bismuth can be found even above sea level, if there are huge mountains.

[Maelstrom]

For clarification (hopefully @Streetwind shows up)...

Density mode does NOT report on chunks.  It reads the heat map for probability of ore generation in an area.  When conducting density searches ignore the numbers UNLESS you want an indication of which adjective description is better - if you have two readings of decent the higher number points in the direction to continue searching.  The first block you break in this mode is the one that determines the reading.  You could conduct two searches with the first block broken being just 2 blocks apart and get a slightly different reading even if both are in the same chunk.

Personally, I take a stack of ladders when exploring an ore reading.  I'll dig a single block shaft down and place a single ladder every other block to preserve ladders.  Since you can hang onto a ladder without an impact to mining speed, there's no reason to dig down MC style (which will use twice as much durability on your pick, btw).  Hanging from a ladder also keeps from dropping into the middle of a mob of your friendly neighborhood drifters.

In my experience ultra-high readings are practically a guarantee of ore to be found.  For copper, bismuth, zinc and tin; you're likely to get multiple deposits within the single shaft and potentially making a maze of tunnels from mining out the different deposits.  Bread crumbing back to the escape tunnel can help keep from getting lost.

 

[Streetwind]

I have been summoned. Your lack of a sacrifice has been noted. I'll put it on your tab

And yes, the prospecting pick in density search mode interpolates across more than one chunk column, weighted by the exact block coordinates of the first block broken. I don't know the exact range.

Confusingly enough, while the prospecting results don't work on a per-chunk column basis, the actual ore generation very much does. The ore map has a resolution of 1 pixel = 1 chunk column, and all ores are given a "you may try spawning this many times per chunk column" value. Each try has a success chance described by the ore map.

For most ores, if you score an "ultra high" reading, you can be very confident that the local chunk column has an extremely good value. Spawn areas rise and fall gradually from one chunk to another, and to even get an "ultra high" from a scan that interpolates across more than one chunk column, they all need to be pretty good. Only a few outlier ores, like gold and silver, actually occur as single pixels on the ore map, and for those it is worth trying to determine which chunk is actually responsible for the reading.

Honestly, in this case, OP has been very successful with density search. I suspect his problems stem not from ore not spawning, but rather from not having a good methodology for what comes after density search. LadyWYT and Maelstrom both adressed this adequately already. I'll add the recommendation of looking up bismuthinite nuggets in the handbook, to see the different stone types that bismuthinite can occur in. Then it's easy to know when it's worth to keep digging and when it's worth to stop whenever the stone type changes while digging.

[Brandybuck]

Chiming in late here, but yeah, you need to switch to node search mode. There are videos showing how to do this.

Bismuth is fairly easy to find. It's the matching sphalerite needed for bismuth bronze that's a nuts. Small deposits that are hard to see against darker rocks.

[Maelstrom]

Ummmm...  Sphalerite is nearly undetectable in granite.  In darker rocks it is as painfully obvious.

[pigfood]

BTW, if you have node search enabled, you can switch the radius in real time using a chat command like:

/worldConfig propickNodeSearchRadius 8

If you have a large radius, locating the actual deposit can be rather frustrating. I use a radius of 8, when I dig an exploration shaft and switch to smaller radius to locate the actual deposit.

[Maelstrom]

Last I checked changing the node search radius requires a relog.