pilot light; spark
Refers to a small flame kept burning to ignite a larger fire, such as a pilot light in a gas appliance, or metaphorically a spark that starts something.
The pilot light on the gas stove had gone out.
live coals for firelighting
Specifically refers to glowing embers or coals used to start a fire, similar to 火種 (ひだね). Often used in traditional or camping contexts.
See also: 火種 (ひだね)
That small incident became the spark of the revolution.
We were careful not to let the live coals go out while camping.
Compound of 種 (たね, 'seed') and 火 (ひ, 'fire'), literally 'seed fire'. The exact historical derivation is uncertain, but the meaning is transparent from its components.