There is much information to be gained by examining and being able to identify rough stones. The skills required are of greatest importance to prospectors and miners. Examining the mineral content of river or stream gravel can indicate the gem-bearing potential of the area, the relative closeness of a nearby gem deposit, and not least, the gem species present.
Factors To Consider
Crystal form: Determination of gem species by studying crystal form (prism, pyramid, pinacoid, dome) enables thte separation of the more valuable gem species from those worth less (distinguishing ruby from garnet). They may be hard, resistant to chemical weathering, and have high enough density to become concentrated in river beds or terraces where suitable traps are present forming placer deposits. Gem species that often occur together in gem gravels include: ruby and red/pink corundum; sapphires of various colors; spinels of various colors; red and reddish brown garnets; zircons of various colors.
Associated gem species: Sometimes gem species occur in common with one another: if the geology and occurrence of similar associations are well-known, then the discovery of one member of a mineral collection can indicate the possible presence of gem materials (sapphire and zircon in the basaltic rocks of Thailand, Cambodia and Australia; ruby and red garnet in the basaltic rocks of Thailand and Cambodia; ruby and spinel in the marbles of Mogok, Burma).
Degree of wear: How abraded is the gem material? Are the crystal faces clean and lustrous with sharp angles, or has the crystal begun to assume a rounded shape? Answering these questions can provide us with an idea of the distance the gem material has traveled from its source rock. The more rounded and fragmentary the material, the greater this distance is likely to be.
Percentage of gem quality material: (a) What amount of the gem bearing gravel is gem quality material? (b) What kind (species) of material is present and in what quantity (how much is present)?
Cost effectivenness: (a) Does the quality of the material justify mining it? (b) Is the necessary machinery (and labor) available?
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