For one word that refers to a jar there are only two occurrences in the Bible, both of them in the same account. But it is a ceramic vessel. The context makes that clear, especially the connection with a potter forming the jar. That it really is a potter is established by other associations of these words in Hebrew.
It is the word בַקְבֻּק (baqbuq), generally translated as a jar.
This is what the Lord says: “Go and buy a clay jar from a potter. Take along some of the elders of the people and of the priests 2 and go out to the Valley of Ben Hinnom, near the entrance of the Potsherd Gate. There proclaim the words I tell you, 3 and say, ‘Hear the word of the Lord, you kings of Judah and people of Jerusalem. This is what the Lord Almighty, the God of Israel, says: Listen! I am going to bring a disaster on this place that will make the ears of everyone who hears of it tingle.’
10 “Then break the jar while those who go with you are watching, 11 and say to them, ‘This is what the Lord Almighty says: I will smash this nation and this city just as this potter’s jar is smashed and cannot be repaired. They will bury the dead in Topheth until there is no more room.'” Jeremiah 19:1-3, 10-11
The passage does not give us an idea what might have been stored in the בַקְבֻּק (baqbuq), doesn’t tell us what it might normally be used for or what it looked like. But it probably should be big enough so that the breaking makes some impact, and also small enough that it can be easily carried. According to James L. Kelso the בַקְבֻּק (baqbuq) is a burnished decanter, a vessel form that became particularly popular in the 7th century (and would therefore fit well with Jeremiah using it).
The following decanter was found at Ktef Hinnom, Jerusalem. The image is from the Israel Antiquities Authority.
A בַקְבֻּק (baqbuq) could well be such a decanter. But the Bible does not give us sufficient information to determine that exactly.