Just anything that he grabs or is it a proper prop?
It is a stone.
It seems to be just a stone. Maybe Strange wanted to distract Thanos while the Avengers were getting ready to attack, or just to do something while waiting for Thanos to come for the Time Stone. There are ruins around, so finding a stone while waiting for Thanos wouldn't have been very hard.
While talking with Thanos, he rolls the stone, and we can clearly see that it is stone. Then, it seems he just dropped it on the ground (looks more like a filming error).