I agree with Pete regarding the 'idea' behind the nut bowl, innovative and practical.
The dimension control on the coasters shows a fine attention to detail preparation.
The bowl part is sycamore and is indeed cut in quarters with one of the quarters stained walnut and glued in a reverse setup to of set the contrast.
A simple jig can be made to secure the bowl while it is cut
A precise cut with a fresh blade makes for a very easy assembly