Buzzarde

Greetings I found this at a archaeological dig site near Lake Thonotassassa. The site is a archaic Indian village/ kill site. The marble was found at the 31" depth. It is 24 mm dia. and is not very round but is very glossy and made of glass. There were no known homesites anywhere near this location, however there was a rail line built around 1905 till 1930s about 400' to the north. This could have been buried in the last 40 years because of artifact poaching in the area? Any ideas? Thanks Bruce