Rune ilvl

[Thrugg]

I was wondering about where a rune X could drop. My concern is about where to find a cham. In others word, is it possible to get a cham in an area level 68 ??

Short answer, no.

1 - First assumption (and question) :
Runes clvl is equal to thier ilvl. For example cham requires a clvl=67, so its ivl should be 67 too.
Is this true ?

This isn't true for any items, and especially (?) not for runes. Runes don't have an ilvl in the usual sense, they are "simple items" and their origin is not recorded.
That isn't really important though, as in general, required level is no indication of where something can be found.

2 - Second Assumption
If area lvl is 68, could any monster of this area drop any item which ilvl<68.
So any monster in stony field (MonLvl13Ex = 68) could drop any item which ivl is 68.
Is this true ?

This bit is more or less right. Monsters in an area of level 68 will drop items of ilvl 68, which is one of the factors involved in exactly what they can drop. More importantly, they will, where possible, upgrade their TC to a TC of level ~68 (as near as they can get) which in this case is the key determinant of what can drop.

Cham is rune 32. This means to drop one, a monster must be able to drop from at least Runes 16 which is the lowest rune TC that contains it.
Runes 16 is only dropped directly by Act 4 (H) Good. The "Good" TCs drop all the runes, charms, jewels, gems and jewellery you see in the game.
Act 4 (H) Good is contained in many monster TCs, but typically require a TC of at least level 78 to drop them. Less than that, and you only get a chance of Act 3 (H) Good which only contains up to Ber.

So, you need to look in at least level 78 areas and above.

Of course, this doesn't really require you to go much further beyond the Stony Field, as the Den of Evil, Hole and of course Pit are all that high.