Always make sure the seller has specified this parameter for every cab they listed for sale. 

For your information, a normal size cabochon 40x20x5 mm could weigh approximately 10 grams. 

A cabochon designed for earring is preferably made smaller and thinner for example 20x10x3 weighing just a few grams. 

We offer some cabs that are thinner than 3 mm. 

Another point to consider, cabochons of moderate translucency would appear more translucent if cut thin, thus enhancing the aesthetic appearance of the earring while still maintaining resistance to breakage due to their smaller size.