Excellent large, dark, and highly textured stone in the image is a type of lava rock, a generic term for several varieties of volcanic (extrusive igneous) rocks formed when lava cools and solidifies.
25 to 28 " x 13 to 15" x 5 to 8" weight my guess 15 to 20 lbs as mentioned I believe has some cement on the bottom but could be removed.
It is flat but large.
Underneath this piece does have some cement. Had it since mid 70s. Is very nice.