1. Yellow sapphire or yellow quartz gem
4. Molten rock that issues from volcanoes
5. Very hard and very precious stone made of crystalline carbon
7. Lustrous gem found within the shells of some molluscs
8. Ornament of precious metal often set with stones and worn as an accessory
10. Rich green gemstone - May's birthstone
11. Small plane surface on a cut gem
12. Transparent deep blue gem
15. Deep yellowish green transparent gem (anagram of 'diopter')
18. (A metamorphic rock (anagram of 'singes')
20. Gem carved in relief
21. January's birthstone (anagram of 'argent')
22. Outer part of our planet, composed essentially of crystalline rocks, shares its name with the part of toast eaten least
23. This stone, much sought by ancient alchemists, was believed to turn base metals into gold
25. Study of the history of the earth as recorded in its rocks