The Fire Crystal Slab is a block added by Thaumic Bases. It is a slab version of the Fire Crystal Block and can be used for stabilizing the Infusion Altar and for decoration.
After researching the way Ordo interacts with blocks in the Crucible you think, that you can start a bigger project. You've found, that by mixing a crystal cluster in a crucible with some ordo, the aspect the cluster is based on and some Fabrico the cluster collapses into a solid form! It is yet to be discovered what the solid form can be used for, but it has saved all the properties of the cluster it was made from. You have also discovered, that by adding Vitium essentia during the process will taint the clusters, resulting in a Tainted Crystal Block. Their uses are yet to be discovered... All blocks can be used to stabilize the Infusion Altar.