TomeNET

(5.17a) Digging - exact calculation                                             
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Your digging power is calculated as follows:
An equipped item that provides (+X) to digging will add 20*X.
If you have a digging tool equipped, its weight is added (eg 25.0 lb -> +25).
If the digging tool is enchanted to hit or to damage, those values are added.
The power value so far is now multiplied by a value between 1 and 2, depending
on your 'Digging' skill (see (7.2)). If the skill is 0, the multiplier is 1, if
it's 50, the multiplier is 2. Otherwise the value is linearly inbetween.
After that, a bonus is added depending on your strength:
Ranging from +0 at 3 over +10 at 18/10, +50 at 18/110 up to +100 at 18/***.

If you don't have a digging tool equipped, your total digging power is halved.

If your resulting final digging power is zero, then it is set to 1, so you
always have at least a tiny chance of digging through the weaker materials
that have a +0 difficulty (see table below).

To successfully dig through certain features, your digging power must be
greater than the result of a random number roll "random(0..x-1) + y":
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Spider web           100 +  0
Rubble               200 +  0
Bush                 300 +  0
Dead tree            300 +  0
Tree                 400 +  0
Sandwall             300 +  0
Magma intrusion      400 + 10
Quartz vein          800 + 20
Granite wall        1600 + 40
Tunneling any other wall types, such as ice walls, and also unknown secret
doors (as long as they're not embedded in permanent wall types) has the same
difficulty as granite walls.

The +y is a fixed value that gets added to the random roll of (0..x-1), the
sum of this is what you have to overcome with your digging power value.
For example you cannot successfully tunnel through quartz if you don't
have at least 21 digging power, since you have to overcome the +20.

Example: If you have 21 digging power and try to dig into quartz you'll have to
be very lucky and hope for the random(0..799) roll for quartz to actually be a
0 so that the result will be 0+20 which is 20 and therefore smaller than your
digging power of 21. So on each turn of tunnelling your chance to succeed would
be 1 in 800 in that case.