I went past them in Covent Garden (London), the store looked very nice, amazing lighting setup!
I asked the staff what kind of stones they were, they answered "diamond simulations" when I asked what kind, if they were CZ? they reluctantly answered no, they are better, they grew them from minerals?
Few quotes from their website: http://www.carat.co/web/en/aboutUs
"we are also certain that you will not find a better alternative than CARAT*. "
"Measuring an impressive 8.5-9.0 Mohs (the scale of mineral hardness), with only diamond measuring the maximum 10 Mohs, our stones are close to sapphire and ruby in Hardness."
Anyone came across carat*?