The old ruby mines are located on the east side of the Pingu-taung (Spider Hill) just below a conspicuous pagoda, the landmark of Kyatpyin.
The principle rocks are leucogranite, graphite marble, graphite-phlogopite marble, ruby-bearing marble and others. The rubies
recovered from the primary rock are generally of good quality, though small. Star rubies
are found occasionally, some of them of fine quality. Rubies associated with other minerals and alexandrite
are found at the nearby skarn.
Ref: Ted Themelis (2008) Gems and Mines of Mogok, p. 245