You must be very careful buying Guineas. There are some really good reproductions out there, so if you buy, buy from someone who knows their Christensens and will stand behind the marble. Clear base is usually the cheapest, then cobalt base and lastly amber base guineas. Torch marbles will usually have a small black dot on many of the individual color flecks. I bought a Guinea from a great seller in Des Moines who took it back without any questions when Sami and Galen Wilcox said it was a really well done reproduction.