Colbalt blue spinel- a decent substitute for Kashmir sapphire?

I've been on the hunt for a decent 'velvety blue' stone to emulate the Kashmir sapphires I've seen, and was wondering if anybody here had compared the two stones to each other in person. Would the refractive index and color be similar enough to pass without lab testing? I'm mainly interested in that velvet glow/beautiful color for a ring project, and an acquaintance has both lab and naturally grown spinel in stock.