Ahh OK Klaus.. that is an interesting question. First though the simple answer.. it is bone.
Yes.. Antler is bone.. a special type of bone as opposed to an ivory, horn, wood, coral or any of the other broad material categories. Compared to other normal mammal bone however.. antler grows much faster. They are also created in a different way and have slightly different mineral content.. all to better suit their primary function, designed for sustaining high impact loading and bending moment without fracture.
These things give antler certain characteristics that differ from normal bones. "Antlers have a similar microstructure as mammalian long bones, composed primarily of type-I collagen fibrils and carbonated apatite crystals, arranged in osteons in the compact bone and a lamellar structure in the cancellous bone... The cross-sectional microstructures in compact bone from antler and bovine femur are shown in Fig. 1(a) and (b), respectively. Osteons (Os), vascular channels (Va), Volkmann’s canals (Vo), and lacunae spaces (L) can be observed in both antler and bovine femur. However, there are several distinguishable differences between the two. Antler consists mainly of primary osteons, which contain newly formed concentric lamellae surrounding vascular channels
(Optical micrographs of compact bone from (a) elk antler and (b) bovine femur (Os, osteons; Va, vascular channels; Vo, Volkmann canals; L, lacuna; I: interstitial bone)
My apologies for getting into such technical detail but I thought Klaus and some others will appreciate a proper scientific foundation for considering the difference between antler and normal mammal bones.