Why did the Navajo Indians use turquoise in their jewelry?

Turquoise is one of the four Navajo sacred jewels or gems. Each is associated with a direction, mountain, color, season, holy people (spiritual beings), time of day and part of life. The directions and mountains are always listed east, south, west, and north. Turquoise is the gem for the southern mountain which is Mount Taylor near grants New Mexico. The Navajo name is Tsoodzil. It marks the southern boundry. It's color is blue. It is covered in dark mists and female rain (soft misty rain). Cougar is the protector. It is also represents mid day, summer, the time of life when you have kids. Turquoise is featured in many parts of the creation stories and has multiple levels of symbolic meaning.

One explanation for silver and turquoise jewelry is that the stone represents the scared and the silver the secular thereby making a complementary dynamic whole symbolically. (Witherspoon)

Here is a quote from the Dine' Bahane' (navajo creation myth)

"In the South they re-created Tsoodził, Blue Bead, or Turquoise Mountain. They adorned it with turquoise. They adorned it with dark mist. They adorned it with animals. They adorned it with light clouds that bring female rain. From two stones they had brought with them they fashioned Dootłʼizhii Náyooʼałí Ashkii, the Boy Who Is Bringing Back Turquoise, and Naadą́ą́ʼ ląʼí Náyooʼáłí Atʼééd, the Girl Who Is Bringing Back Many Ears of Corn. They were created to reside there forever."