During the Edo period, the word "Giyaman" was used to refer to diamonds (kongōseki). It is said to be a corruption of the Dutch word diamant or the Portuguese word diam o. Originally, the term referred specifically to diamonds themselves, but over time it came to be used more broadly to refer to glass products in general, including gemstones like crystal and cut glass processed with diamond techniques.
As early as the late Muromachi period, Dutch traders introduced glassware manufacturing techniques to Japan, including glass vessels such as sake cups, bottles, and bowls known as bīdoro. These glass items were often confused with "Giyaman," and the term came to collectively denote glass products other than flat glass sheets. Consequently, the strict distinction between Giyaman and bīdoro was lost.
Additionally, bīdoro is believed to be a corruption of the Portuguese word vidro, meaning glass.