Gold Coin, Made Of 21 Carat Pure Gold, Jewish Heritage, Ancient Egypt

Not so. A typical 'school boy' testing kit includes acid for silver, 9ct, 14-22ct and steel/platinum.

It's not a coin. I would assume it to be a modern zodiac medal. The stars arranged around the scorpion clearly indicate it is meant to be interpreted as the star sign Scorpio.

It almost certainly isn't "Jewish"; as noted above, the Seal of Solomon (which usually has a dot or star in the centre of the hexagram) is an occultic traditional symbol that predates the common use of the Star of David by the Jewish community.

If you google "gold scorpio medal" or "gold scorpio coin", you will find millions of little tiny gold scorpion medals very much like this one. They all have some kind of loop or attachment to be worn as a pendant, which this piece seems to lack; it may have been held in place by a bezel instead.

The black colouration worries me; 21k gold shouldn't turn black like that naturally. I suspect it may have been artificially "blackened" to try to make it look older.

Regarding my previous comment, I was speaking from personal experience working as a gold buyer for a number of years. My point was that typical gold kits include various reagents but nothing specifically for 21k. A very weak reaction to 22k acid might be interpreted by some as "21k" but most would default to the highest test unequivocally passed. I didn't think it necessary to discuss all possible items included in various metals testing kits. We used XRF analysis when warranted but on a day to day basis, I found acid-stained fingers preferable to radiation exposure.