Currently BarSeries/ColumnSeries are rendered like a "glass" gradient and PieSeries/DonutSeries like a "roundedBevel" gradient. Expose an OverlayGradient property that lets you control the gradient (e.g. "glass", "roundedBevel", "sharpBevel", "none"). For the time being the properties can be set through the chartObject:
JavaScript:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script>
			function pageLoad() {
				var chart1 = $find("<%=ColumnChart1.ClientID%>");
				//Set glass gradient to first series
				chart1._chartObject.options.series[0].overlay = { gradient: "glass" };
				//Remove gradient from second series
				chart1._chartObject.options.series[1].overlay = { gradient: "none" };
				chart1.repaint();

				var chart2 = $find("<%=PieChart1.ClientID%>");
				//Set sharpBevel gradient to pie series
				chart2._chartObject.options.series[0].overlay = { gradient: "sharpBevel" };
				chart2.repaint();
			}
		</script>
	</telerik:RadCodeBlock>
ASPX:
		<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Transitions="true">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Series with Gradient">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="23000" />
							<telerik:CategorySeriesItem Y="10000" />
							<telerik:CategorySeriesItem Y="16000" />
						</SeriesItems>
					</telerik:ColumnSeries>
					<telerik:ColumnSeries Name="Series with no Gradient">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="35000" />
							<telerik:CategorySeriesItem Y="10000" />
							<telerik:CategorySeriesItem Y="20000" />
							<telerik:CategorySeriesItem Y="17000" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>
		<telerik:RadHtmlChart runat="server" ID="PieChart1" Transitions="true" Width="450px" Height="450px">
			<PlotArea>
				<Series>
					<telerik:PieSeries StartAngle="90">
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="Purple" Exploded="true" Name="Internet Explorer" Y="18.3" />
							<telerik:PieSeriesItem BackgroundColor="Orange" Exploded="false" Name="Firefox" Y="35.8" />
							<telerik:PieSeriesItem BackgroundColor="Green" Exploded="false" Name="Chrome" Y="38.3" />
							<telerik:PieSeriesItem BackgroundColor="Blue" Exploded="false" Name="Safari" Y="4.5" />
							<telerik:PieSeriesItem BackgroundColor="Red" Exploded="false" Name="Opera" Y="2.3" />
						</SeriesItems>
					</telerik:PieSeries>
				</Series>
			</PlotArea>
			<ChartTitle Text="Pie with Gradient">
			</ChartTitle>
		</telerik:RadHtmlChart>