Rashi on the original braitta in Shabbat 66b appears to identify it with the Old French קוטאנ"א, קונטנ''ה or קונטיינ"א - "contiene", related to the English "contain"; see below citation of transcriber of Maharshal that the bell-like stone contains a second stone. (Alternatively, perhaps the reference is to what in French is currently called cornéenne or, in English, hornfels which, interestingly enough, has also been associated with bells due to its acoustic properties.)
Rav Shmuel Loew Machatzit HaShekel OC 303:12, after citing the Taz, cites a transcriber of the Maharshal who, apparently referring to the mythological "aetite" or "eagle stone" (hattip @interested) claims that the Even Tekuma is a stone that naturally contains another smaller stone (rattling) in a hollow inside of it like a clapper in a bell:
ודע דמ"ש בש"ע יוצא' באבן תקומה כתב הט"ז היינו מה שקורין שטערין שו"ס ובספר א"ר כתב בשם המעתיק ביאורי רש"ל לסמ"ג שהוא אבן שיש בו חלל ואבן קטן בתוכו כענבל בזוג וכן נברא עכ"ל
Know that [regarding] what is said in the Shulchan Aruch [that] "She may go out with a tequmah stone", the Turei Zahav wrote that this is what is called "shterin shus"*. And in the book Elya Rabba, he [Rav Eliyahu Spira] wrote in the name of the transcriber of the explanations of the Maharshal on the Sefer Mitzvoth Gadol that it is a stone in which there is a hollow in which there is a [second] small stone, like a clapper in a bell, and so it was created [i.e. naturally]. End Quote.
Presumably, the inner stone represents a fetus secure within its mother.
See also Rav Qafih (Commentary on Laws of Shabbath 19:14:34, cited here) who reports his personal awareness of the continued traditional use of such a rattling stone, presumably for this purpose, in his time in Yemen:
ופרש"י אבן תקומה אבן שנושאות אותה נשים עוברות שלא יפילו וקורין לה קונטאנא בלע"ז. ולעז זה איני יודע אבל מכיר אני את האבן שהיא אבן חלולה אטומה באופן טבעי, ובתוך חללה אבן קטנה המקשקשת כאשר מניעים אותה, וארבע כאלה בגודלים שונים היו למשפחות מוסלמיות בעיר צנעא שהיו משאילים אותם לכל דורש יהודי או ערבי ללא שום תשלום אלא גמילות חסד.
Rashi explained: tequmah stone - a stone that pregnant women carry so that they not miscarry and it is called "contene" in the foreign language (Old French).
This foreign word ["contene"] I do not know. But I recognize the [tequmah] stone. It is a hollow stone that rattles when moved. Four such [stones] of differing sizes were owned by Muslim families in the [capital] city of Sana'a. [The Muslim families] would loan out [the stones] to any one who sought them, whether Judaean or Arab without [requiring] payment, just as a kindness.
See also Rabbenu Chananel on Shabbath 66b whose general terminology may suggest that the specific mineral type of the stone was either unknown or irrelevant. See also Kaf Hachaim whose version of Rabbenu Chananel seems to suggest gravel or a pebble, though that may just be a comment on the size, rather than the composition.
Alternatively, assuming like the Taz that the stone is a שטערי"ן שו"ס (shooting star - hattip @fred) and that his meaning is a meteorite, it is possible that the reference may be to meteoric iron or natural stainless steel. (Notably, eagle stones have also been made of iron, at least more recently.) However, a close reading of his actual chosen words might suggest that he did not himself directly identify the stone with the celestial phenomenon:
ונרא' דהיינו מה שאנו קורין שטערי"ן שו"ס שנושאים נשים מעוברות
It would seem that this is what we call "Shooting Star" that pregnant women carry.
It's worth pointing out that the braitta is not necessarily endorsing the efficacy of the practice. Rather, it appears to be classifying it as an adornment instead of a burden, and therefore permits its wearing in the public domain on Shabbat. Nonetheless, if it floats your fancy, try for a (meteoric) iron bell;)