Can you continue to re-roll magic shield to get JMOD?
Are there diminishing returns?

3 Perfect Gems (of any type) + 1 Magic Item = 1 New Random Magic Item of the Same Type
You can use this formula to get another chance at a Magic Item of the same type. The ilvl = ilvl. This means to use 100% of the ilvl of the input item. It's important to note the difference between "the ilvl is set to a fixed number" like what happens in some recipes and "the ilvl of the output is exactly the same as the ilvl of the input" as is the case here. So if you're cubing for +skills Grand Charms, for example, you want to use Charms starting from late Nightmare, not ones that dropped in the Normal Blood Moor. You can use this formula (with an Amulet) to create a +2 Skills Amulet. And you cannot Gamble one before Character Level 84. Be warned however, that the chances of getting that on an Amulet might be very low (considering everything is available) and it might take a lot of tries. Rare Items cannot be used with this formula to create new Rare Items. You can also use this on Charms and Jewels.

Its possible but the odds are horrible. Better reroll gcs

So there is not diminishing returns?

One can re-roll same shield over and over right?

Never any DRs, just keep re rolling

You're probably thinking of formula for rare items where it lowered ilvl. And there is another formula using SOJ to raise it again

Just buy a jmod

You do see my gold count right lol

chance to roll jmod is 1:14220
so you need on average over 42k pgs
you can't afford that either

I agree however the point was for information not gold spending advice.