'g' is for grams for example - 100g is 100 grams in weight not a counted amount.

'app' is short for approximately - which gives the customer a guide as to how many pebbles for example they might get for the weight ordered i.e 100g of 20mm sized round pebbles app 23.

 

This means for example - 100g in weight of 20mm sized round pebbles gives about 23 pebbles (this depends on the individual weight of each pebble in 100g of weight so the figure is an approximate guide)

Larger sized pebbles, marbles and chippings etc. weigh differently so there are different amounts.