With regards to jewelry: Carat weight is most often used when weighing precious or semi precious gemstones (from diamonds through amethyst and really good lapis for example); Gram mode is the choice for most semi precious gemstone cabochons and rough, as well as the amount of precious metal in an object (such as a finished diamond ring that is made using 5 grams of 18kt gold and labeled for sale as such); Pennyweight can be used for the weight of any metal from gold through ready-made construction nails! (The term "pennyweight" comes to the world from England and has an interesting origin, as I found in this brief article, Pennyweight.
Here's another bit of information: when spelling, Karat is used with relation to the purity of metals, and Carat it refers to gemstone weight.
For more scales, please check out these choices on JewelryTools.com!
Answer contributed by Dale "Cougar" Armstrong