Below is an instruction that describes a task. Write a response that appropriately completes the request in Chinese. Please answer in Chinese.

### Instruction:
 一条由三种珠子组成的项链有40颗珠子。有7颗紫水晶珠子和两倍于紫水晶珠子数量的琥珀珠子。第三种珠子是绿松石。这条项链中有多少颗绿松石珠子？


### Response:
 琥珀珠子有7 * 2 = <<7*2=14>>14颗。
绿松石珠子有40 - 7 - 14 = <<40-7-14=19>>19颗。