The BitMEX .ZECXBT Index tracks the Zcash price every minute. The Zcash price is calculated from the last price at Poloniex. The .ZECXBT index price is shown on many pages as the current Zcash price.
This index is composite, which means the price is built from multiple sources. See the "Composite Index Breakdown" below for information on the constituents. If an exchange is to lose service and no trades are printed for over 15 minutes, BitMEX services may automatically remove that constituent from the index until trading resumes.