Complex sentence: The field is usually composed of one or more tinctures (colours or metals) or furs, except for in cases, which are rare in the extreme, in which the field is not a tincture, but may be a landscape.
Split into simpler sentences: The field is usually composed of one or more tinctures (colours or metals) or furs. In extremely rare cases, the field is not a tincture, but may be a landscape.