Small delay in securing sources of unique blue gemstone (8)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Small delay in securing sources of unique blue gemstone S (=small, of sizes) + [ U <nique> B <lue> (“sources of” means first letter only) in TARRY (=delay, linger)]; a star ruby is one that shows a star-shaped luminous figure as a result of reflected or transmitted light