Our Mexican turquoise beads (not always available) usually come from the Pino Chueco ("Crooked Pine") and Campitos mines in Sonora, Mexico. We occasionally carry AA-grade natural (unenhanced) turquoise from the Sleeping Beauty Mine in Globe Arizona (not always available).
A few stones are frequently called "Yellow Turquoise." We sometimes stock "yellow turquoise" but would like to make it clear that nothing on the market is truly yellow turquoise, or purple turquoise. Both are simply descriptive names common in the gemstone industry.
Yellow Turquoise beads from Rings & Things are usually a jasper or serpentine, and are not dyed or enhanced. This is a hard yellow stone with brown-to-black (and occasionally red or green) matrix. The black matrix in yellow turquoise is often attracted by a magnet, and is probably hematite.
Another variety of yellow turquoise is "soft yellow turquoise", which is theoretically a pale natural turquoise from China's Hubei province, dyed a deep yellow. However, we suspect it is actually dyed magnesite, dolomite, calcite or even limestone. When we stock this form of yellow turquoise, we are careful to mark it dyed or enhanced, so you know to take extra care when cleaning and storing it. Purple turquoise is generally the same thing. (Turquoise does not naturally occur in shades of purple.)