Yeah... Too bad the effect is so subtle; barely shows when the stones are worn: you need a thin slice getting light from the back The orient of pearls is very obvious by comparison. What I like in the rainbow of agates is that the effect has depth - as it does on pearls and MOP, not just a thin layer as in the 'rainbow' fractures in crystalline quartzes, etc.

GIA has published some striking pictures of transparent, colorless agate slices with relatively strong rainbow effect. I have never found such material on offer (those might have been just very thin slices of the usual pale yellow - just dunno.)