what causes leathery leaves?

i got a couple plants that turn dark purple in flower, after the color change the leaves get leathery like a magnola tree's leaves. while green they are normal. i try to keep ph @ 6.5-6.7, if anything they where underfed but it is general hydroponics maxi grow, but while the same thing with just this strain using viagrow 10-10-10 the whole grow. all my other plants seem to be ok other that learning my new setup. this one has me stumped but i would like to keep this strain as it is easy to clone.

Can you post some pics of the whole plant and close-ups of the leaves and stems that are leaves you're worried about?

Tanning beds. You should see my sister in law.

Low Humidity and/or Overfed....try backing off the feeding and maybe use water only for the next feed or 2, will take up to 10 days for the leaves to return to normal

low humidity maybe as the cold months it is 25 to 50 depending on out doors temp. as far as pics i got some from a old grow and have already chopped these down. i know this is the only strain i have this problem with. i'll see on some pics later. by the way it was supposed to be sour diesel regs but all have been female and from what i'm told sour diesel doesn't turn a dark purple to almost black.

Leathery leaves imo is nitrogen Def to much nitrogen,
Hard to diagnose without more info.

maybe i shouldn't say leathery as they are more like a magnolia tree leaves. shiny, hard and thick. still trying to find a couple pics that show what i'm talking about.