Visible differences between obsidian chemical types in the region are principally in terms of nodule size, fracture characteristics, glass color, and cortex. While nodule size and fracture characteristics are believed to have been important determining factors in explaining which obsidian types were circulated widely in prehistory (Burger, et al. 2000: 348), glass color is more of visual aesthetic issue. The obsidian sources in the south-central Andes are predominantly black or grey. Obsidian from the Chivay source is often a transparent grey and banded, and Quispisisa obsidian sometimes has a red coloration, although Burger et al. (2000: 314) state that the Quispisisa type is visually indistinguishable from Alca obsidian. As with many goods with discrete places of origin, these visual differences communicate information about the spatial origin of the stone that would have visually linked the material with regions and socio-political groups to knowledgeable viewers. To Brooks (1998: 452;1997) the transparency of Chivay obsidian was a reason for its wide circulation in prehistory. Others have commented on the transparency of the material including Giesso (2000;2003), and Burger et al (2000: 296). Giesso (2003: 368) observes that archaeological and ethnographic evidence from the Andes indicate that "transparent elements were viewed as mediators between different cosmological worlds". Further study may permit evaluation of observed patterns in the contexts of obsidian use that are linked to color.