How heavy were the Ten Commandments?

I know it isn't relevant until Ki Tissa, which we won't read for another two weeks. (Or it was relevant to the daf from 3 weeks ago), but I was just wondering...

How heavy were the two tablets that Moshe brought down from Mount Sinai? I mean in weight, not in importance.

The Talmud (Nedarim 38a) tells us:

R. Johanan said: The Holy One, blessed be He, causes His Divine Presence to rest only upon him who is strong, wealthy, wise and meek; and all these [qualifications] are deduced from Moses. Strong, for it is written, And he spread abroad the tent over the tabernacle; and a Master said, Moses our teacher spread it; and it is also written, Ten cubits shall be the length of the board. Yet perhaps it was long and thin? — But [it is derived] from this verse: And I took the two tables, and cast them out of my two hands, and broke them. Now, it was taught: The tablss [sic, tables] were six [handbreadths] in length, six in breadth, and three in thickness.

There are different opinions as the length of the Biblical Amah. It is somewhere between 48cm (according to R' Chaim Naeh) and 57.66 cm (according to the Chazon Ish). So the tablets were each between 2.88m and 3.5m on each side and between 1.4m and 1.75m thick. At a minimum this is almost 12 cubic metres of stone. This is over 18 tonnes! And Moshe carried two such tablets down from Mount Sinai.

Can someone please check my calculations and see if I did this right? And convert it into old fashioned numbers for the Americans who don't speak metric? Does this make sense?

I know the Talmud says that Moshe was strong, but he was REALLY Strong!!! And this is the requirement for someone to be a prophet! Wow! I'd better get down to the gym quickly if I want to have any chance.