1. Sundial ate Al₂O₃SiO₂ polymorph from Spain (10 letters)
7. Initially United Nations (2)
8. A claim the copper mineral makes (9)
11. Wager in time (3)
12. Get rammed into old vehicle (4)
14. Right in or maladroit fault (6)
16. Put us ever in work (3)
17. Lying or seated in, it covers the hills with thorns (5)
18. Not extra (5)
20. Tore in to can raw stuff (3,3)
23. Return no Vedic philosophy into old English county (5)
24. Plainly gaseous (5)
25. Backed us in soap (3)

Clues down

1. Mailed Nan about a variety of garnet (9)
2. Toe riled up in 5 down (8)
3. Act age over sale of semi-precious stones (4,6)
4. Back us, it seems, in clothing (4)
5. Suitors inn rearranged underground igneous phenomena (10)
6. Formerly extra without tar (2)
9. In saga gone past (3)
10. Relax in grease (4)
15. Look less OK (2)
19. Conduct heavy metal (4)
21. A clinging sort of lass (3)
22. Turn on negative (2)
23. Initially deep sea (2)