A bit of online research this weekend revealed that they're probably Akro marble glass. Akro was a company that originally founded in Akron, Ohio around 1910 or so to make marbles. Akron is about an hour from where I grew up, and my parents, grandparents, aunts, uncles and cousins were all from that area. The company eventually moved south and added toy dishes to its product line, before going out of business in 1951. These dishes don't have the Akro trademark, but they have the hallmark design and coloration.
These always seemed mysterious and fanciful found objects, the lost treasured possessions of some unknown girl who lived in our house before we did, and so a bit magical.