Final answer:
Gem-quality minerals in zoned gem-bearing pegmatites are typically found in the core zones, where the crystalline structure is most complex. These zones are rich in elements like chromium, giving rubies and emeralds their colors. The location and quality of gems are influenced by the pegmatite's chemical composition and the host rock's temperature conditions.
Step-by-step explanation:
The general physical morphology of zoned gem-bearing pegmatites often shows gem-quality minerals located in distinct zones. In many cases, these zones form during the final stages of the crystallization of a pegmatite. The crystalline structure tends to become more complex towards the center of the pegmatite body, and it is here that collectors may find the highest quality gemstones, such as rubies and emeralds.
Gemstones in pegmatites are typically formed in an environment rich in elements like chromium, which gives these gems their characteristic colors. Chromite presence as Cr³+ impurities within the colorless metal oxide lattice is responsible for the rich colors seen in gem-quality crystals. The location and quality of gem deposits within a pegmatite are influenced by a variety of geological factors, including the chemical composition and temperature conditions of the host rock.
In the zoning of a pegmatite, the most desirable gemstones are often found in the core or in pockets within the pegmatite that had the proper conditions for large crystal growth. These gems are typically embedded in a matrix of other minerals that have crystallized out of the magma at different stages of cooling. Pegmatites with well-formed gem crystals are highly sought after for mining and exploration due to the value and rarity of the gems they contain.