I haven't seen anything more explicit than the Introduction to Ceylon blog posts, where Gavin does mention that only the natural and float types are built-in.

I haven't seen anything more explicit than the Introduction to Ceylon blog posts, where Gavin does mention that only the natural and float types are built-in. It's not actually clear to me that the Decimal type is base-10. The word decimal is fairly often (mis)used to mean number representations with a decimal point.