I looked into this last year partly for fun and partly for viability and chose to abort after a few experiments.
The recommended thickness of concrete is minimum 50mm and considering granite is 30 -35 mm you can imagine the weight. I have a polished piece outside my back door in black 600 * 1000 and it's heavy....really heavy. Looks the dogs though.
There are two ways of casting them...in situ and off site. In the off site method you form a mould from contiboard and cast upside down giving a flat surface when cured. Before you pour the concrete you can stick things like coins and metal objects to the contiboard so they are exposed in the finished product. You can also put in decorative aggregate and grind and polish afterwards to expose it. The concrete can be dyed during mixing or stained afterwards in all sorts of weird and wonderful ways.
The in situ method involves forming a cast and pouring on top of the carcasses but this has several drawbacks in that it's almost impossible to get a flat surface, it's potentially very messy and the polishing is very wet & time consuming.
Generally the off site method is preferred. For both methods various additives are required to reduce shrinkage and surface cracks. These products aren't available in this country (I may be wrong there) as well as the dye which has to be pure pigment to achieve a decent colour. In the US you can buy all-in-one bags you just add water and mix.
Another product is known as microscreed which is applied thinly by trowel to almost any surface and you can then polish it. Once again very hard to get in this country.
The main reason I aborted the whole thing was to market something like this in the UK which is unknown to the british public would be extremely expensive. I mean if someone said to you ' concrete worktops' you'd think they were mental.
I reckon when the technology comes along where you can make these things and don't need a fork truck to install them they might catch on. But until then they're purely top end stuff. There are some companies here do them and it's worth a look at their websites.
You can make other things too like sinks, shower bases,fireplaces, table tops, bartops.