Fantasy novel wasn't the first in its series. There is a group of people with elemental abilities that are known as Sheperds. but if you kill one you absorb their power. one of them can make his skin turn to stone at will. Read around 2014.

[BirdandCatLover]

i read this book around 2014. the flighted species offers rides via a (uncomfortable) container that they carry. the queen ended up visting her brother who was a evil king i think, and he imprisoned her. she was pregnant, but gave birth while in her cell. the copy i had had a picture of a poker card on the cover, and my guess is the book had like 300-350 pages.
has a non-human species of tall, flighted humanoids one named "T"

[Becca]

The Dark Sea Annals series by Wayne Thomas Batson looks like a match. You say it wasn't the first in the series but what you describe does sound like the first book, The Sword in the Stars - it has the queen giving birth in a prison cell and a playing card on the cover:

Both books have the Shepherds and the skin-turning-to-stone thing:

And the skin on Jonasim’s arm...there was no other way to describe it...the skin had turned to stone.

Alastair gaped. The Halfainin would have many Shepherd-gifts. The prophecies had declared this. But not this Shepherd-gift. Not stone-skin.