This technique is often used in Siam Sterling Jewelry and there is a whole collecting area of vintage jewelry referred to as Siam Sterling Nielloware. However, the process is not unique to just Siam and Thailand. Certainly, a large amount of the Nielloware jewelry does come from this country, but the Niello process has been used for hundreds of years dating back to the 13th century and before. Some researchers believe that Niello was originally produced in Egypt, as far back as the 4th century B.C.
Siam sterling Nielloware pieces are marked in a wide variety of ways. Markings may include: Made in Siam, made in Thailand, Siam Sterling Silver, or Siam Sterling 925. Normally, the markings are made directly in the metal instead of on cartouches. The name of the country can't be used to definitively date the piece. Even though Siam was the name of Thailand prior to 1939, the markings for Siam jewelry continued to be used for many more years. Here are pictures of a few of the markings.
Strictly speaking the Niello process results in a black background. There are many sterling silver pieces from Siam which use other colors such as blue, green, red, pink, white and yellow. While these are collectible in their own right, they use a less demanding enameling procedure. Designs of Nielloware often displays Nekkala, the Goddess of Lightning from the Hindu legend Ramayana.
There are other designs as well, although they are not as commonly seen, including monkeys, elephants and other symbols from Thai culture, folklore, religious lore, and spiritual beliefs. Here are pictures of two Nielloware designs as well as the blue enamel sterling silver design which is not a true Nielloware design, but resembles it. The blue leaf, although not Nielloware, does use the Nekkala motif that one often finds on Nielloware pieces.
Much of the Nielloware jewelry available in the USA came from husbands and boyfriends who were on leave during the Vietnam War and sent gifts of Siam Sterling jewelry to their girl friends and wives. A lot of this jewelry was completely hand made. While not especially valuable as jewelry pieces, Nielloware jewelry pieces are wonderful keepsakes and often will hold sentimental value for the bearer.