What is the Age of Emerald Gemstone?
Among the oldest emeralds that have been cut as gems are those from Sandawana in Zimbabwe, where the pegmatite veins containing the emeralds (Figure 1.26) have been dated to about 2600 million years (Ma), which is the late Archaean (see Table 1.1). Emeralds from a very similar geological setting also occur at the Menzies mine in Western Australia and these rocks also have been dated to about 2600 Ma.
About 2565 million years later, and in completely different geological conditions, the emeralds of present-day Colombia were being formed in hydrothermal veins in the mountains now known as the Andes. Between the Tertiary (35 Ma) deposits in Colombia and the Archaean (2600 Ma) deposits in Zimbabwe, emeralds have formed at many different times and in many different places. In the Kaduna-Plateau states of Nigeria, for example, emeralds were formed in two distinct episodes: the first was associated with the ‘Older granites’ between 450 and 600 Ma ago, and the second associated with the ‘Younger granites’ between 140 and 190 Ma ago. From these examples, two general observations can be made:
• Emerald can form in similar geological conditions at the same time in different parts of the Earth’s crust.
• Emerald can form in the same region of the crust at different times. It is important to note that the ages assigned to emeralds, for example at Sandawana, are based on analyses of associated minerals in the host rocks, and the reliability of any dates depends on correct interpretation of the rock textures. This way of estimating ages can be extended to many other gem minerals occurring in pegmatites but not to diamond in kimberlite.