Sardonyx is a type of microcrystalline quartz, specifically onyx, banded with browns, reds, or yellows instead of black. Onyx and sardonyx are types of agate, which is the gemstone of those with the sign Gemini. Sardonyx is closely related to onyx, sard, and carnelian. SARDONYX ACADEMY? It is a prestige school located at the Philippines. All of the pupils are rich. But one day, everything has changed. One by one died. The question is: WHO ARE THE KILLERS? Date Started: October 12, 2016 Date Ended: October 22, 2016 Highest Rank: #36 in Horror [12-13-16] Late Sardonyx University.