One of the best slurper caps made. The fully faceted version is most current with everything else on sale:
Ultra pure quartz with machine precision lensing and optical polish for perfect seals.
Insulative. Quartz is significantly more insulative than borosilicate, and the handle allows for safe operation.
Improved airflow - the cone both augments rotational energy and deflects vapor from going directly to the stem.
Widely compatible - these caps fit 16-30mm tops. Deeper cones may interfere with valves.