When I buy high quality cabs of semi-precious stones, I almost
always buy them by carat. The exceptions to that are generally
stones that are either matched cabs or slabs in specialty shapes,
items that are carved, and lower-quality "training" or "commercial
grade" materials. In those cases, I buy at a per-piece or, for
commercial-grade, per-hundred price. Items that aren't necessarily
"stone" (like bone, shell, slag, glass, etc.) are also usually priced
per item or per set.
Even in the per-piece or per-hundred pricing, though, it can be good
to know the carat-weight, so that you can do an apples-to-apples
comparison of price... sort of like the price/unit conversions you
see on the grocery store shelf that always show price/oz or price/lb,
regardless of the package label or size.
Hope this helps!