Embossed building block. It's very hard.

Additional information:It is a very strong block, being completely immune to explosives. It was originally known as, “Portal Block” it was used to create a portal to flamas. This feature was removed in void phantom's invasion, and it's name was later changed in the “Set sail to the ocean!” Update, to embossed brick. The only way to obtain it as of now is by chance when trading 2 Amber with the altars found in the quatzacloatl temple.