Yes, there really is a difference between 35 mil and 57 mil chipboard:
The thinner leaves (0.92 mm) have one delicate white surface that presents much better color when scribbled with fat-tip colored markers. The thicker frame (1.45 mm) is ordinary kraft chipboard which seems much more durable and looks terrible when colored.
Although it may be a case of gilding the dandelion, a durable kraft frame sets off the petal colors and, being slightly thicker, may also protect them from immediate destruction by sweaty drinks.
We’re talking artsy coasters here, not cheap disposable junk. Right?