Citrines come in many shades of yellow from dark whisky to light lemon. From the picture, it looks like citrine, but testing is required to know for sure. A Presidium tester will differentiate the quartzes from citrines and glass (and other gems as well) but is expensive at almost $400 (of course there are numerous gemological tools you can use). A simple non scientific test is to hold the gemstone by the girdle and touch the table to your cheek. Topaz tends to feel cooler to the cheek than quartz. If you have other known topazes or quartzes, try this test then check your new stone. This may give you a better clue until you can have it tested.