The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.
A cursory web enquiry suggests Black sunstone β "a plagioclase feldspar variety that ranges from a dark gunmetal gray to a true, deep black", and a 'quartz/graphinite' called Black aventurine β "while green aventurine is the most famous, quartz can form in dark gray and pure black varieties." Searching those on specialist minerology sites might yield results.
I can imagine Black opal, which has quite a variety of patterns, approaching an appearance of aventurescence, though I have never seen any. {The poster formerly known as 87.81.230.195} ~2026-56420-06 (talk) 03:28, 8 November 2026 (UTC)
Goldstone, while aventurescent, is not a true gemstone but can pass as such. Since it can be made in basically any colour, one can make black goldstone gems, like one depicted here β actually a very dark blue. Β βββLambiam 11:47, 8 November 2026 (UTC)