The spelling of the word "heather marbles" follows the conventional English spelling system. According to the IPA phonetic transcription, the /h/ sound is produced at the beginning of "heather", followed by the /ɛ/ vowel sound. The /ð/ sound is used for the letter combination "th" in "heather", and the /m/ sound is used for "marbles". The /ɑ/ vowel sound is used in "marbles". Overall, the spelling of "heather marbles" accurately represents the pronunciation of the word in English.