These metal balls have a perfect tight fit that won't ever leak. With the bottle inverted, the weight of the steel ball creates a seal preventing oil release. A quick shake allows the release of the oil for a smooth and even application every time The durable brushed stainless steel cap screws on securely and protects your essential oils while maintaining an elegant look and feel Certain oils & chemicals do not interact well with cheap plastic balls and possibly ruin the rolling mechanism. Stainless steel roller balls do not erode and will continue to roll on smoothly every time Size: high 8.8 cm, wide 2.1 cm Capacity:10ML/pc Material: thick glass ,Package include 5pcs 10ml bottle + free 1pc 3ml free dropper + free 1pc clean cloth