Diablo – Is the amount of damage absorbed by diamond skin proportional to weapon damage

So this is a bit of an interesting issue. To answer this, we need to take a closer look at the damage formula:

Base Damage =
        (weapon damage range + bonus damage from gear)
        × bonus from primary attribute^†
        × other class bonuses from skills.

Ability Damage =
        Base Damage (as above)
        × % Weapon Damage listed on the ability.

^†1 primary attribute = +1% bonus damage

For the purpose of this question, we can ignore ability damage and bonuses from skills.

Understanding the Formula

Base Damage has two main components. Weapon Damage (this is the range you get, i.e. 4-6 for many starting weapons) and Primary Attribute bonus (this is 1% for every Primary Stat you ahve. In your case, Intelligence).

Because they are being multiplied, a small weapon range gets less benefit than a large one. 4-6 damage times 200% damage (from 100 intelligence) is only 8-12, whereas a weapon with 6-10 goes up to 12-20. The higher your weapon's average damage, the more valuable the bonus from stats is.

Applying our Findings

In your case, you've got a ton of Intelligence, which is great, and a really low damage weapon, which is less great. This means that really low damage is getting multiplied by a large value, which is still, unfortunately, in your case, a really low final value.

The take away here is that primary stat is not worth more than weapon damage. You need both to maximize your damage, which means you'd probably be best served by dropping your 20 dps weapon and picking up something nicer. As a comparison - My Wizard has a 33 DPS weapon, and she is only in Act III of Normal.

Diamond Skin scales exclusively with character level, though as mentioned it absorbs damage after reduction is calculated. The only way to increase its damage absorption directly is to use the rune that doubles its base absorption value.