I got this from a friend many years ago and assume it's from one of many "silver treasures" found on this island, from some caliphate ~1000 years ago, but I would like to know a bit more about this specific coin. I don't know arabic and I know half the coin is missing, but if anyone know more than me I'd be glad to hear!
I can give you half an answer or maybe a little more. The general type is Samanid. The only clear identifier is the mint name Samarqand. Otherwise, we have a partial date digit which appears to be arba' ("four") and the last letter of the name of the ruler's father, "...d". Only two rulers fit all the criteria, Isma'il bin Ahmad, AH 279 to 295, and Nasr II bin Ahmad, AH 301-331. Both struck coins at Samarqand with five dates possible, 284, 294, 304, 314 and 324. Of those, only the coins of 324 have the extra letter (?) or ornament in the lower obverse field, which is evident on your coin. https://www.zeno.ru/showgallery.php?cat=13737