An 18mm cone piece is a type of glass accessory that is used with a water pipe or bong. The cone piece is the part of the bong that holds the dry herb and is inserted into the bong through the downstem. It is 18mm in diameter, which is slightly larger than the standard 14mm size and is considered a larger cone piece.
The 18mm glass cone piece typically has a male joint and is used in combination with a female joint on the bong or water pipe, creating an airtight seal. The cone piece has a bowl on the top where the dry herb is placed and a hole on the bottom for the smoke to travel through.
An 18mm cone piece is usually made from borosilicate glass, which is a type of glass that is resistant to thermal shock and can withstand high temperatures. It's also easy to clean and maintain.
The 18mm cone piece is generally considered as a larger size, it's useful for those who want to smoke more herb in a single session or for those who like larger bowls. The larger size means a glass cone piece 18mm will need to be cleaned less often then a 14mm cone.