Assessing stone samples - colour. You need to have stone that has some consistency in colour. You will get variations in shade but if there is a combination of colours (think Sodalite) then you have to ask, 'how much will I have to throw away when I cut the stone into 10mm (3/8") tesserae?'
Also, too many veins and it will not cut well enough, it will split across the line of the vein.
Test part of the stone with wax and see what the colour comes up as. Look at the stone after waxing and you need to be able to categorise it in shade and colour. If you find this difficult then the stone may not be good enough to use.