This is Mopani driftwood. They are dense and naturally will lay on the tank bottom.
Driftwood is not only decorative, but is valuable when keeping plecos and other wood-loving catfish. They actually consume the wood over time. It appears to aid their digestion and helps keep them in breeding condition. When putting it into tanks simply to keep plecos happy, it's wise to buy it by weight. The heavier it is, the longer it lasts before they consume it totally. We've actually had grow-out tanks that were heavily populated with bristlenose, where 3 lb pieces were completely eaten within a few years.