In question 19901 we quoted a fatwa from the Standing Committee on the zakaah of jewellery that is made to be worn. They said that the more correct of the two scholarly opinions is that jewellery that is made to be worn is subject to zakaah. It should be noted that it is the fact that the gold is made to be worn that makes it subject to zakaah; it does not have to be worn all the time.
Based on this, the fact that the gold has been left with your mother for safekeeping does not alter the fact that it has been made to be worn, and that it belongs to you, so you have to pay zakaah on it because it reaches the nisaab or threshold above which zakaah is due, because the nisaab for gold is 85 grams. See question no. 2795 and 64.
Based on this, this gold is subject to zakaah which you should pay each year, at a rate of 2.5 percent of its value.
And Allaah knows best.