Reposhing from my vintage jewelry collection!! Black Czech Ring 1930 1940. Inside is stamped Czech. Vintage 1930/1940 black Jet Glass Filigree ring. Stones are clear, and I see no chips or scratches. The ring size is adjustable, so you may wear on whichever finger you desire. Jet is a type of lignite, a precursor to coal, and is a gemstone. Jet is not a mineral, but rather a mineraloid. It has an organic origin, being derived from wood that has decayed under extreme pressure. ... The adjective "jet-black", meaning as dark a black as possible, derives from this material.