etymology - Character vs Charm - Pronunciation

Is there a rule to understand how the group "Cha" has to be pronounced?

"Character" sounds with a hard first syllable, while "Charm" sound softer, but I don't find how to tell which sound to use before earing someone saying the word. It could be because of the double consonant "rm" vs "ra"? Or is it just a matter of knowing the rule for every single word?

EDIT: More specifically, let's talk about UK English pronunciation.

Answer

I already dealt with <GH> pronunciation variation here; <CH> is a more interesting situation because it involves borrowings from familiar European languages, rather than languages written in other orthographies.

The grapheme <C> goes all the way back to the Semitic glyph gimel, the third letter of the original alphabet: 'aleph 'cow', beth 'house', gimel 'camel', etc. 'Aleph represented a glottal stop, a phoneme the Greeks didn't need, so they threw it away and invented vowel letters (which Semitic writing didn't need so much and didn't use).

So they made alpha a vowel letter. Beth became beta and /b/ is /b/, pretty much the same thing. Gimel /g/ became gamma /g/, and the letter still had the same camel-like hump.

When the Romans borrowed Greek letters, alpha became <A>, beta <B>, and gamma <C>. But it no longer meant /g/; it got devoiced to /k/; <C> always represents /k/, in Classical Latin (Medieval Latin is quite another matter).

And that's the last time that <C> always represents anything. When Latin split into the Romance languages, and Latin writing became a standard for other languages, <C> split into many varieties, depending on the original histories of the various languages, what sound changes had occurred when to which one, and which words had been borrowed into which languages (before or after the sound changes). Some of these variations acquired new spellings as <CH>, because the grapheme <H> is often used to differentiate letters.

From the standpoint of English, the various pronunciations of <CH> include:

• [x] (a voiceless velar fricative, which does not exist in Modern English;
    but which does occur in German Loch, Scots loch, Hebrew /ləxayim/, and Russian /xoroʃo/)


• /k/, a voiceless velar stop, often a subsitute for [x], as when Americans say Loch Ness /laknɛs/;
    but also in words borrowed from Italian, if <CH> is followed by <I> or <E>,
    and also in words borrowed from Latin, no matter what follows <CH>.


• /ʃ/, a voiceless alveolopalatal sibilant, in words borrowed from French.


• /tʃ/, a voiceless alveolopalatal affricate, in most native English words;
    and also in words borrowed from Spanish.
    (This is probably the most common pronunciation)