The above is a magnified part of the original image below captured by the Curiosity rover on Mars on sol 1729 (June 17, 2017).
From the added information with that raw image it can be estimated that the distance between the lens and the target was about 14 cm.
Below another image of the thin piece with surroundings in the sunshine.
Click (or double click) on the image for magnification.
Asking about the chemical composition of this piece of material on space exploration SE, it appeared from the answers that most likely it is CaSO$_4$, perhaps the hydrated form gypsum.
I've seen many other raw images from Mars, but not anything like this.
What could this thin piece of material be, does it resemble anything on Earth ?