18K gold is a high quality gold used for jewellery, it is 75% gold and 25% other metals. Alloying pure gold with other metals is necessary to improve work-ability, scratch resistance and strength to hold gemstones safely. Pure gold is 24K. Also used for jewellery is 14K (58.3% gold), 10K (41.6% gold) and 9K (37.5% gold).
For the technical people (marked K/24K) x100 = %gold
A cabochon is a gemstone which has been shaped and polished smooth as opposed to faceted. Usually an oval or round shape with a domed front face and the back generally flatter or slightly domed. A cabochon shows off the beautiful colour of less transparent or opaque gems and also light phenomena like stars, cat’s eyes and opalescence in opals.
The carat is the standard worldwide unit of measure for the weight of gemstones. One carat = 0.2 grams. 1 gram = 5 carats.
The name carat comes from the seed of the carob tree native to Mediterranean regions. The seeds were a consistent weight around 0.2 grams and long ago were used to weigh gemstones.
Oysters secrete mother of pearl which coats the surface of pearls and the inside of the shell. A mabe pearl (ma:bei) starts as a dome shape of mother of pearl on the inside of the shell, cut out and closed on the back with a flat piece of mother of pearl also from the shell, effectively making a pearl cabochon.