It's an earthenware marble that has been dyed or painted. I've found jars of them. They're commonly found on 1890-1920 sites.
"EARTHENWARE
Earthenware marbles can be divided into two classes: brown-bodied and white-bodied. The latter class originates from earlier periods than the former. A third class, yellowware, comprises a very small portion of the earthenware marble category.
Common Brown-Bodied Earthenware
Common brown-bodied earthenware marbles are usually referred to as "clays" or "commies." They were manufactured from low-fired brown or red clays, and depending on the amount and type of impurities always present in the clay, would assume a post-firing color of red, brown, gray, or tan. These marbles are very porous and rapidly absorb water placed on them.
These marbles were produced in vast numbers in both Europe (mainly Germany) and America. Their date of production is between the mid 1700s up to the late 1920s or even 1930s. They are often found in their original boxes, most often from Germany but even found in Christensen Agate Company boxes.
Brown-bodied earthenware marbles have been found on archaeological sites dating to the 1600s in England. However, these are isolated incidences and do not represent commercial manufacturing, which appears to have begun until the middle of the following century. By late in the same century they became common enough to be found in significant numbers at archeological sites in Europe as well as America during this period. Early clay marbles were molded by hand, as evidenced by many of them being out-of-round and even having fingerprint impressions, while after the invention of a marble-shaping machine in 1859 they were made more perfectly spherical.
In America, these marbles were probably made locally by immigrant Moravian potters in Bethabara, North Carolina, around 1756-1773. They may also have been produced at potteries in Westmoreland County, Pennsylvania between 1795-1840. In 1889, several potteries in and around Akron, Ohio, began producing "commies." These were apparently made is such mass amounts that the flow of these marbles from Germany was effectively halted.
Many of these American clay marbles were not specifically used in children's games. Large numbers of them were utilized in a number of different manners, particularly in oil cans and oil pipe lines to clean out paraffin buildup.
As mentioned, manufacture of common brown-bodied earthenware marbles began in Akron, Ohio, in 1889. They were first made there by S.C. Dyke and Company, and shortly thereafter by that company's co-owner's brother, Acton L. Dyke. It was Acton Dyke who apparently introduced painted, or dyed, clay marbles in 1890, around the same time he also applied for a patent for machinery that could produce 600 marbles per hour.
The following year the two brothers consolidated their companies and operated as the American Marble and Toy Manufacturing Company until 1904. They soon had their machinery producing approximately 1000 marbles per hour. These marbles were sold both dyed and undyed; the dyed examples were most frequently red, green, yellow, and blue. Some were dyed in solid colors while others were speckled with one or more colors…" Nonglass Handmade Marbles ID
You never know what you will find in a plug. I once found a clay figure head that was pre-Colombian. They use to put them in their gardens for better crops.