One metal used by the jewellery trade that became vital for the war was platinum. Platinum is a rare metallic element that is more expensive than gold. It is strong, doesn’t corrode or tarnish but is not easy to use because it melts at extremely high temperatures. There is evidence to show it was used in Egyptian, Pre-Columbian and Inca jewellery. The Spanish explorers came across it in South American and discarded it. Generally, platinum needs to be combined with other metals such as copper, rhodium or iridium to create a workable alloy. In the 17th and 18th centuries, scientists in Europe, particularly in Spain and France, attempted and, in many cases succeeded, to find processes through which platinum could be used. The Napoleonic Wars stopped progress but in 1803, a British scientist discovered a process to make a malleable alloy which could become mainstream. However, it was not until the mid 19th century that it began to be used in Victorian jewellery, as furnace heating temperatures increased. Jewellers like Fabergé and Cartier began to use it for high end diamond jewellery.

Platinum was used in the production of munitions and WWI saw restrictions being placed on the use of platinum in jewellery as it was needed to mark armaments. By 1917, the British government had taken over control of all platinum. The US had initially worked with jewellers such as George Kunz of Tiffany & Co about sharing the use of platinum but in mid 1918, the government prohibited the use of platinum for jewellery.
Jewellers returned to using gold and silver. However, gold was also in short supply and as a result, jewellery became lighter in weight. The little buckle ring below is a much pared down version of buckle rings produced before the war. The knot ring with diamonds pictured further down the blog looks ornate and heavy but it actually quite hollow and very light.

While many jewellery workshops were closed during the war, as the staff were conscripted into the armed services or into defence work of some sort, some jewellers remained. Fashion magazines advised women that wearing jewels was ‘bad form’ but black and white jewellery became popular. This didn’t mean a return to the heavy Victorian jet mourning pieces though. Onyx and enamel was used as well as some jet. Louis Cartier first developed the stylish spotted Panthere watch using onyx in 1914.
Lockets, with photos of serving soldiers or with a lock of hair, were also acceptable. Double-sided lockets, like the one at the top of the post, were common.

Sweetheart jewellery emerged as a new category of jewellery. They were generally inexpensive jewellery mementos bought by soldiers serving overseas, or items made by them in the trenches or barracks. These were sent home to girlfriends, wives and mothers. They featured hearts a lot as well as flags and stars. An offshoot of sweetheart jewellery was patriotic jewellery. In Australia, this consisted of brooches and pins depicting symbolic images such as the rising symbol (as shown in the General Service Badge or the Australian Army), the southern cross, kookaburras and koalas. Some lovely examples can be seen at this Australian War Museum blog site https://www.awm.gov.au/articles/blog/commercial-sweetheart-jewellery

References:
Ariana Bishop, ‘”The Importance of Being Jeweled:” Patriotism and Adornment in the United States during the First World War’, The Journal of Dress History, Vol 3, Issue 1, Spring 1919
Richard L. Lael and Linda Killen, ‘The Pressure of Shortage: Platinum Policy and the Wilson Administration during World War I’, The Business History Review , Winter, 1982, Vol. 56, No. 4 (Winter, 1982), pp. 545-558