No information anywhere on thorn

- Increased thorns damage (obvious)
- Increased elemental / cold / fire / lightning / phys / chaos
- Gain #% of damage as extra phys / chaos / elemental / cold / lightning / fire
- %increased damage against enemies with ailments / stunned / dazed / etc
- Adds # to # thorns damage (gear)

Thorns has no base crit but there are nodes and modifiers on jewels that provide
+ #% to thorns critical hit chance (watch the phrasing) This is BASE
Once you have base thorns crit it scales from any source of
- increased (global) critical hit chance
- increased critical hit damage
That is as long as it doesn't state "spell" or "attack"
The easiest way to scale it is to get the boots that give 25% and the talent that gives 6 together with 1 or 2(corrupted helm) with stag idols. That way you'll already have over 60% crit.

# to # thorns damage can roll on body armors, belts, and shields, there are also some unique items for other slots.
The persistent buff "Scavenged plating" also gives A LOT but you do need to keep up the stacks by breaking armor constantly.

There are also 2 ways to scale thorns with mana stacking,
The first is by using the body armor "Icetomb" and the second is by using a support gem on channeled melee skills of which I found 2 reliable ones
The first is: "Resonating shield
The second is: "Fury of the mountain"
Mana stacking is a good way to get a lot of base thorns damage.

There are some ways to utilize thorns damage without YOU needing to be hit first.
- Spell gem: Lightning storm, this adds your thorns damage to the base dmg.
- Support gem: Caltrops, this turns a projectile skill into a trap and adds thorns
- Support gem: Brambleslam, on slam skills it puts vines on the ground which deal thorns damage when a enemy walks over it
- Ring: Prized pain, this applies your thorns damage on hits that you heavy stun with.

Thorns is applied sepperatly from hits
Example, If you've stacked a lot of stun buildup and you'll stun a enemy with resonating shield while using the prized pain ring you will do 2 instances of damage, that from the skill and then that from the thorns. So if you've got something like ailments or armor breaking going you can fit your Resonating shield skill with something that prevents you from ailing or breaking and your thorns will still apply it provided that you've got a source somewhere that allows your thorns damage to do so (like the Warbringers - Anvils Weight).

Thorns per default only reflects against melee hits, if you want to reflect against ranged attacks and spells as well the only way to do that is by using the helmet "Crown of the pale king"

Not confirmed but 90% sure: When a enemy hits you it seems like your thorns damage is first applied before it calculates the damage that mob does on you.
If your thorns damage is high enough to kill a enemy that attacks you in 1 hit then it seems like their damage from single/straight up hits is not applied so having very high thorns damage seems to serve as a damage reduction layer as well. AOE's or attacks that do multiple hits/continue after death do still hurt.