Some people use the terms "promise ring" and "purity ring" interchangeably, but they're different.
Purity rings, or chastity rings, have a different history and motivation behind them than promise rings.
For instance, putting lapis, opal, verdelite, and emerald together on a ring spells the word "love" — the first letter of each stone's name creates the acrostic.
If a couple was not in the best place financially to buy a home or start a family, they would display their intentions of marriage with a promise ring.
They were named as such because they were engraved with lines from romantic poems or other short messages.
Some examples of inscriptions on discovered posy rings are "united hearts death only parts" and "let us share in joy and care." A couple centuries later, during the Georgian and Victorian eras, acrostic rings became popular.
The gemstones in these rings were arranged to spell out words, like a secret message.
Many people nowadays ascribe whatever meaning they want to promise rings.
For instance, some women give promise rings to their friends to indicate their platonic bond with each other.