Big cut stone

Hello I’ve just noticed this stone which is in very good condition and big, 6.65 ct , I have been looking at pictures on sites which show the difference between diamond and sapphire and mossianite , even cz, but it might as well be Chinese because I can’t tell at all , I just can’t seem to get it, so I’m posting this in the hope a serious gem expert happens to pop on and sees it. And more importantly has some idea.