What are the prices of Granito tiles?

I have been reading through this thread and I commend everyone for the useful information shared. It's priceless. I am in the unenviable position of trying to undertake a construction project while living outside the country. I know it presents huge risks but I still wanted to get the project started as I plan to get back and manage the project. Since pictures replace 1000 words, above is a picture of the foundation of standard residential apartments I am putting up; nothing fancy. The area of the land is approx. 230 sq m. The plan s to build 3 two bed-roomed apartments and 1 one bed-roomed apartment per floor. The ground floor will have shops + res. units. The footers have been built and 4 layers of stone and about 30 foundation columns have been erected. Contractor plans to back-fill the interior space with the soily material your see on the right side as hardcore. Up to this point, I felt my contractor was doing OK, until I saw what he plans to use for "Hardcore". My question is, is this material proper to use in place of real hardcore which should be larger gravel stones? In my budget, I have hardcore, ballast and sand catered for. Why would my contractor use this material instead of the budgeted items? This has raised doubts in me and I have stopped the construction at this point. Also what would (have) been the correct rod (chuma) size to use on raising the foundation columns; y12 or y16? He says he used Y12 in some sections and Y8 in others. Is Y8 used in any part of the foundation? I am planning to send a structural engineer to evaluate the work done so far and if necessary recommend amendments. Is this a feasible arrangement. If so can I get some leads?

Thanks @Mosby. I would be relieved if the murram in the picture can be used for back-filling and the hardcore placed above it. I will verify again where any Y8's may have been used, and if any have been used in raising the columns. Do you also mean that without the structural engineer's drawing I cannot get an evaluation of the structural integrity of the foundation?

I thought it was mandatory that all buildings have a Structural Engineer's drawing which gives details regarding the chumas, foundation, slabs, columns, load bearing, balconies, staircases, roof etc etc, before they are approved. The structural drawing also guides fundis on the quality, quantity, sizes etc of the various materials to be used at each stage.

I have independently verified that NO Y8's have been used in erecting the columns, they are Y12's. My contractor has also indicated that he is using the services of a structural engineer, so what I want to do is hire my own to confirm that I am getting the proper service. Since I have paused the construction, I have time to travel and gauge everything before I decide to proceed or not.