The Hebrew word for "Life" is חי CHAI, which has a numerical value of 18.

Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life as 18 is considered a blessed number because of it is the numerical value ofחי

In Judaism, in the Talmud; Pirkei Avot (5:25), Rabbi Yehudah Ben Teime gives the age of 18 as the appropriate age to get married ("Ben shmonah esra lechupah", at eighteen years old to the Chupah (marriage canopy)). (See Coming of age, Age of majority).

Stone size:
M- Model HPS-B (16mm) / (0.63Inch)
L- Model HPS-D (19mm) / (0.74Inch)
XL- Model HPS-E (22mm) / (0.86Inch)
Recommended size :HPS-B (M) 
Stone: Natural Jerusalem stone 
Silversmith's work: Handmade
Material: Silver 925
Type: Pendant
*Each pendant comes with 2 chain a silver 925 chain necklace (45cm) and a Leather necklace with metal clasp and gift box . 
** The letters and Symbols are carved in the stone
*** Stone is all Handmade Cutting and polishing there is a small change between the different pics of jewelry