Amethyst

Crystal Vaults links amethyst to the third eye and crown chakras and describes it as a stone of spirituality, meditation and creativity that also "bears the logic of temperance and sobriety". According to the site, it is the zodiac stone of Pisces.

The site also tells a legend: Bacchus, the god of wine, angry over an insult, decreed that the first person he met would be devoured by his tigers. That turned out to be a maiden named Amethyst, on her way to the shrine of Diana. The goddess saved her by turning her into clear white crystal, and Bacchus, regretting his cruelty, poured grape juice over the stone, giving it its purple color. According to Wikipedia, this legend was invented by the French poet Rémy Belleau in the 16th century. In other words, it is much younger than the stone.

Chemistry: silica, SiO₂. A purple variety of quartz.

Mohs hardness: 7. No cleavage; it breaks with a conchoidal fracture.

Specific gravity: 2.66.

Crystal system: trigonal.

Color cause: iron in the crystal, turned purple by natural radiation. The color can fade in strong light, and turns yellow or brown when heated.

Sources: Brazil, Uruguay, Bolivia, Zambia (about 1,000 tons a year), Canada and others.

Achlamah is the third stone in the third row of the High Priest's breastplate (Exodus 28:19; 39:12). The Greek Septuagint translated it amethystos, which is where the common identification with the purple stone comes from. But it is not the only identification: according to Wikipedia, the Hebrew name may come from an Egyptian word for red jasper, and the JPS English translation renders achlamah as crystal.

Targum Onkelos translates achlamah as "ein egla", a calf's eye. Rabbeinu Bahya explains that it is the stone called in his language "cristal", and that its virtue is to strengthen the heart "so it will not grow faint when going to war". He derives its name from a root meaning strength, as in "restore me to health and revive me" (Isaiah 38:16).

There is no agreement on the tribe either. According to Rabbeinu Bahya, Gad's name was engraved on the achlamah. According to Bamidbar Rabbah (2:7), it was Naphtali's stone, and his banner was the color of "clear wine whose redness is not deep". A wine-colored banner for a stone the Greeks thought protected against wine. According to Targum Jonathan (on Exodus 28:19), the third row bore the names of Gad, Asher and Issachar, so the achlamah, the third stone in the row, matches Issachar.

The crystal website Crystal Vaults assigns amethyst to the tribe of Dan. We did not find this in the sources we checked.

Amethyst is the birthstone for February and the gem for the 6th and 17th wedding anniversaries.

According to Wikipedia, Anglican bishops wear an episcopal ring often set with an amethyst, an allusion to the Apostles being described as "not drunk" at Pentecost (Acts 2:15).