Christensen Agate "World's Best Guineas" rank among the most prized machine made marbles. Fakes are seen more often now, which is not surprising given the fact that a mint Guinea sells online for an average $350. They are always transparent-based with clear glass, but sometimes the base is amber or cobalt blue. Very rarely the base may be red or green. They can be found in sizes up to 15/16". Guineas have splotched and stretched multi-color surfaces, typically of light blue, light green, orange, yellow, lavender and white. Usually the whole surface of the marble is covered in these colors but sometimes there are large clear gaps.