The answer to one of our crossword clues the other day was limpid. Limpid means clear and bright and is used especially of a liquid. It also means transmitting light , particularly transparent or bright, or able to be seen through with clarity and is used in that context with regard to crystals.
Limpid can also be used metaphoricaly to mean transparently clear or easily understandable; as in "he writes in a limpid style".
An alternative word of similar meaning is the delightful sounding 'pellucid'.
Katie aged 6 months
16 hours ago