The most likely explanation of the marble and cement cylinders is therefore that they are construction materials that were being transported by ship when, either by shipwreck or design, they came to rest on the seafloor off Entrance Point. However, I really do not see anything the pictures that look like fluted marble columns although present at the barrel site. But even with granite is present, it would mean that a ship carrying granite and marble, instead of just marble, along with cement got into trouble and had to dump stuff overboard or was shipwrecked. As a result, evaluation as to what these stones are and their significance is impossible to do. The simple presence of scattered pieces of granite fails to be proof of the existence of a former Atlantean city given the documented abundance of historic trash present in the area.
Granite is a very easy rock to identify in hand specimen.
It is something that first year geology student should be able to do. That a laboratory had to identify it for you is revealing of the level of geological expertise available among your group of divers. In fact, the vast majority of the cylinders consist of concrete instead of marble. Again, the presence of numerous, documented concrete barrels along the shore of Bimini Island indicates that by dumping or shipwreck, the nearshore zone of these islands has been littered with historic debris. Unless you have some way of dating or tracing the source of the marble and granite, it will be impossible for to argue that these granite and marble represent the remains of any prehistoric cultures.