Which term is associated with the appearance of gemstone sizes suited for mounting?

The term "calibrated sizes" refers to gemstones that are produced in specific, standardized sizes that make them easier to use in jewelry mounting. This standardization is especially important for jewelers, as it ensures that the stones fit perfectly into pre-made settings without requiring additional modification. Calibrated sizes help maintain uniformity across the market, making it simpler for designers to select stones that will work seamlessly with their designs.

In contrast, options like boulder opal refer to a specific type of opal that is found in ironstone, and does not relate to the sizing of gemstones. Total color absorption is a concept connected to how a material interacts with light, but it doesn't pertain to gemstone sizes or mounting suitability. Blue sheen describes a visual effect seen in some gemstones, particularly labradorite, and is also unrelated to the concept of calibrated sizes. Thus, the focus on the standardized sizing aspect distinctly highlights why calibrated sizes is the correct answer.